Because the assassin has access to the Ki focus, enchantment is left to the ki meaning your only selection is in the weapon die and properties you'd like it to have.
Things you should be thinking about
- Weapon die size: how big of a punch do you want to pack, and what is it going to cost you to pack that big of a punch?
- Weapon proficiency Bonus: How much of an impact with taking a +2 weapon over a +3 weapon make on your accuracy (do the DPR calculations and figure it out, don't leave this to hypotheticals).
- Feat/Power support: While a lot of your feat support should focus on the ki focus, if you've got some specific things in mind for a specific weapon group then you should choose weapons in that group.
- Variety: You probably have the ability to quickly switch weapons, maybe you want that Glaive for reach, but sometimes need the accuracy of the rapier or the punch of the flail. This is the kind of versatility granted by a ki focus, take advantage of it.
- Story/Character reasons: think about your character, what kind of weapon do you picture him with? Is he the type to be bristling with blades and weaponry? or is he more subtle and favor easily concealed weapons? Does it change from battle to battle? This is probably the major source of your weapon choices (since most of your special stuff is driven by your ki focus).
By default as an assassin you get access to
- Military heavy and light blades
- Simple one-handed melee
- Simple ranged
There is a pretty extensive list of weapons here, particularly in the heavy and light blades (rapier, falchion, broadsword, glaive, longsword, greatsword). I'd start with grabbing a couple from this list (definitely rapier/long sword and probably a glaive, maybe a greatsword just to have a nice punchy option). Then grab an item or two off the simple one handed melee list, although most of the options are trumped by heavy blade options, however if you've got a story or feat/power issue that would provide a good reason to take something off the simple one handed-melee list, go for it.
The Beloved of Valarian, as you have noticed, isn’t really intended for Paladins. You can get in (and wind up with both a unicorn and another mount, which could itself be a unicorn), but you don’t progress any of those abilities, which means stagnating your Smite, your spells, and that mount that you’re interested in.
Really, the class is for Rangers to become quasi-paladins, because you can just
Be a Paladin, have a Unicorn, worship Valarian, call yourself Beloved
The primary class feature of the Beloved of Valarian class is the unicorn companion. Well, a Paladin can just have one of those.
In the Dungeon Master’s Guide, page 204:
A paladin of 6th level or higher can use a [...] unicorn [...] as a mount.
So instead of getting a not-unicorn special mount at 5th level, get a unicorn at 6th level. You count as one level lower than your actual paladin level for the purposes of your special mount’s features.
Then worship Valarian, and you’ll effectively be one of the Beloved. A reasonable DM should have no problems with you calling yourself one/being treated like one in-character, either.
This is actually better than what a Beloved of Valarian gets, because your Special Mount improves with level, while the Beloved’s does not aside from the Telepathic Bond and Celestial Charger class features. Paladins instead receive an Empathic Link, though, and the Celestial template doesn’t do much to an already-intelligent unicorn. Despite not having the Celestial template, the unicorn is “from the celestial realms.”
Otherwise, you’re lacking a couple of +2’s to wilderness-y skills; you can easily get these from feats if you desire them. Timelessness, Uncanny Dodge, Nature’s Understanding, and Wood Repulsion aren’t very high-impact features, though the Timelessness is a nice one from a flavor perspective. One solution would be to be a race that gets Timelessness anyway; another could be to just ask the DM for it since it’s not really going to change the game, just change your character’s outlook on life.
The hard ones are ethereal jaunt and mass baleful polymorph. They’re both just once per day, but they’re also both reasonably powerful and fairly flavorful. These you would simply have to accept you aren’t going to get without actually taking the Beloved of Valarian class.
Unless your DM agrees to work with you:
Were I your DM, I’d work with you to build a feat chain for high-level Paladins of Valarian to get these features. Maybe something like these:
Beloved
Requirement: Must be a paladin of Valarian with a unicorn as a Special Mount
Benefit: You gain the Timelessness quality, add Knowledge (nature) and Survival and paladin class skills, and add the following spells to your paladin spell list:
1st: calm animals, charm animal, delay poison, detect snares and pits, entangle, eyes of the avoral, longstrider, pass without trace, speak with animals
Special: If you turn away from the faith of Valarian, you lose this feat and any feats that require it. You regain this feat if you return to the faith (including any atonement that may be necessary).
Because you can’t get the unicorn mount until 6th level, you can’t take this feat any earlier than that, either.
Entrusted
Requirement: Beloved
Benefit: You may, once per day, become ethereal as if by the ethereal jaunt spell cast by a sorcerer of your paladin level.
You also add the following spells to your paladin spell list:
2nd: barkskin, hold animal
3rd: heart’s ease, neutralize poison, remove disease, water walk
Since it requires Beloved, you don’t get this until level 9, the same level as a Beloved of Valarian. Also, you don’t need special rules about sharing the spell with your mount, because you have the Share Spells feature.
Spear of Valarian
Requirements: Entrusted, ability to cast 4th-level Paladin Spells
Benefit: Once per day, you may cast a special version of the baleful polymorph spell that affects all evil creatures within 30 feet of you (as a 10th-level druid). Affected creatures are transformed into harmless Diminutive mammals (rabbits, squirrels, or the like) or Fine vermin (nonthreatening spiders, centipedes, or the like). All affected creatures are transformed into the same type of creature. See the baleful polymorph spell description in the Player’s Handbook for details.
You also add the following spells to your paladin spell list:
4th: blinding beauty, commune with nature, freedom of movement, spear of Valarian
The requirement of 4th-level Paladin Spells puts this to 15th level; a bit earlier than when a Beloved of Valarian gets it, but reasonable enough.
These allow you to be a paladin, while earning some of the special features that Beloveds get in a timely fashion, without making them “freebies.”
Best Answer
Flavour over Fluff
The key to understanding 3.5e/PF classes is that while they have evocative names (Rogue, Fighter, Assassin, Archmage) and a big pile of writing at the front that says it tells you what they do, often the mechanical effects of the class are not in line with the fluff - and another class might make a better version of the concept you are going for than one with a name closer to the concept.
For example, if a player came to me saying they wanted a character with innate magic they didn't know how to use gifted by dragon blood, and were struggling to control, i'd say Warlock without a second's thought. The Sorcerer class is word-for-word the fluff of what they are describing - but the mechanical abilities are extremely difficult to theme as 'innate, not-controlled magic' - it takes a skilled roleplayer to do so, and one who knows their way around the spell list. Warlock, though, is blasting people with raw magical energy and a few tricks that themselves are weird and not entirely what you might want. It's almost the ideal 'guy with magic who has no idea what he is doing' class, if you ignore the fluff entirely.
Fluff is the writing on a class that has no mechanical effect. As only the mechanics of a class actually translate into the game - your character's story and explanation for his powers supercedes what is written on the class - if you are concerned with flavour over mechanics, oddly, the first thing you have to do is look at the mechanics of any class you are thinking of applying to your character. As those are the things you can't change.
While many people are fine to use the existing fluff of a class, people heavily concerned with flavour will often want to change things up, refluff, or rewrite it to suit their specific character. As the only things you can't change are mechanics, those are the parts to pay attention to - because they are the ones that can stab your flavour in the back.
How To Make An Assassin In PF
Ninja. Ninja has a very eastern feel to it's writeup, but if you look at the abilities it grants, you quickly see there's nothing eastern about the effects. A shadow warrior concerned with stealth and striking swiftly from darkness, the ninja is much more like an assassin than the rogue - but a specific kind of assassin, not one who strikes from every direction and works the social aspect, but one who relies on physical and magical ability to climb up the side of an impassable castle and kill a king in his bed before disappearing into shadows and smoke just as the palace guard burst in.
Rogue. Rogues have the option to pick up some ninja abilities, as ninjas do rogues, but the cross-pollination is not as heavy as you might think, as the iconic (and defining) mechanical effects of the class remain class-specific. Rogues have the option to be sneakier, and dirtier than ninjas, which gives them more power due to the ability to have an indirect, or 'Face' like approach, but gives them slightly less raw 'teleporting through shadows murdering people'.
Vivisectionist Alchemist. This is the most assassin-like base class. You kill people, have a deep and enduring knowledge of anatomy, and use poisons. You also make magical potions that you use to empower yourself to kill people. It's the closest to famous fantasy-assassins, from the Witcher to Durzo Blint to FitzChivalry or Chade. Knowledge of secret herbs in fantasy = alchemy, and it and poison are hand in hand.
Mixing or matching these classes with a splash in some others could work, notably Fighter, Swashbuckler, Barbarian, and to a lesser extent, Inquisitor/Magus, both to represent a higher level of martial ability, or add more 'tricks' up your sleeve. Note that Magus/Inquisitor add martial ability, as does fighter - swash and mostly barbarian add tricks.
Overall, those three base classes are what you are going to be spending levels in to be a fantasy assassin. Vivisectionist gives you the most bang for your buck, but the other two are also fine if you know what you're getting into and dance past the trap options built into both classes.
Theoretically, a monk could be an assassin, with a decent level of monk mixed with rogue, and relying on Stunning Fist and feats that change/boost that ability, but that's a very specific flavour.
Prestige Classes.
Assassin. Assassin's a bit of a poser, because it does it's very best to straitjacket you into acting a certain way as hard as it can. But mechanically, it does only a very few things for you. Sneak attack progression, death attack, true death, and hide in plain sight. The first is self-explanatory, and most 'sneaky' prestige classes are immediately noped out of because they don't grant it. Death Attack is.. interesting. We'll get back to it. True Death is completely boring - so many low level spells, even alchemist's fire, do similar things. There are mechanisms for this not tied to 4 levels in a prestige class. Hide In Plain Sight is great, and fits your fluff perfectly - sadly, it requires level 8, and Assassin requires level 5 to get into - so you're getting this at level 13, aka, well into the game if not by the time the game is over.
Death Attack is really what we come back to. There are optimization resources for magic items, feats, and abilities that increase the DC - most of them are 3.5e. The existing DC will not be very high. Even if you pimp out your Int, and enter Assassin as soon as possible, no monster will die to it except caster-types - and no fighter-type npc will die to it, unless they are not wearing rings of resistance or the like at all. It's a huge let-down when you try this, and it doesn't work, so if you want it, you'll need to really invest in it, not just your character's resources, but also your time in researching how to improve it. It's a weak ability, so it's no small potatoes to try to improve it. On the gripping hand - poisoned dagger full attack with sneak attack from the shadows will likely kill anything that would die to that DC in the first place. So that's a question you have to ask yourself. What is this actually giving me?
Shadowdancer. This class is important because, while it requires 3 crappy feats to enter, it grants Hide In Plain Sight at first level. If it gave sneak attack progression (it doesn't), it would be a shoe-in. The abilities are good, and suit an assassin, especially once you strip the 'agent of the darknesses!' fluff from them. But it doesn't add to your murder ability, and that will start to hurt you as you level up. And the good abilities don't come thick and fast enough to make up for that. A 1 level dip costs you 3 feats and 1 level - but it lets you hide anywhere, and that is invaluable. This class + Assassin at the same time would be the ideal assassin prestige class.
I looked at all of the other prestige classes that seemed 'sneaky' but they were, to put it mildly, crap. Boring mechanics, and boring fluff. I really don't see any that even look slightly good.
I see three major options for you, to make an assassin in pf (as opposed to just a sneaky person) -
Vivisectionist Alchemist 5/Shadowdancer 1/Vivisectionist Alchemist 14
This gives you your magical poisoner/assassin, ala Durzo Blint or Chade. You can dip part of your later Alchemist progression into Assassin to pick up Death Attack, but it's that question again. Alchemist makes you progressively stronger as you get up in levels, enough to keep pace with casters. Rogue 3 gives you Improved Uncanny Dodge and Improved Evasion when combined with Shadowdancer 2, but probably isn't worth the dip.
Rogue 5/Shadowdancer 1/Rogue 14
This gives your silver-tongued, dark-eyed social assassin, who enters the enemy's bedchamber disguised as a maid. The lies and intrigue make you somewhat like the old 'Monster' field agent concept, with a soul as black as tarnished silver. You don't need to dip Assassin with this build, as several Rogue advanced talents are similar to Death Attack, the 'knockout blow' and 'deadly cocktail' being the most obvious. You can actually strip shadowdancer out of this if you want to spend the feats elsewhere - you rely less heavily on stealth than the other two builds, and can pick up Hide In Plain Sight from an advanced talent.
Ninja 5/Shadowdancer 1/Ninja 14
These builds all look pretty similar. And that's because in PF, very few prestige classes are worth it. Ninja has an advanced 'ninja trick' called 'Assassination' that uses word for word the same text as Death Attack. They also get many things that help get into position to assassinate, that the assassin doesn't get.
This is actually the closest to a knife in the darkness, a mundane assassin, that you'll find.