At level 3, warlocks gain the Pact Boon feature, and one of the options is Pact of the Blade. One of the benefits of the warlock’s Pact of the Blade is the ability to conjure any melee weapon the warlock likes, and for the warlock to be proficient in that weapon:
You can use your action to create a pact weapon in your empty hand. You can choose the form that this melee weapon takes each time you create it. You are proficient with it while you wield it. This weapon counts as magical for the purpose of overcoming resistance and immunity to nonmagical attacks and damage.
This received a lot of attention when discussing monster-only weapons like the ice devil’s spear, but developer commentary nixed that combo, barring perhaps if you get proficiency elsewhere and become legitimately Large yourself.
Without such weapons, though, this feature looks rather difficult to leverage: the game rewards specializing, but if, for example, you build around a high Dexterity, non-finesse weapons are basically useless to you. If you instead multiclass with fighter and take the great weapon fighting style and the Great Weapon Master feat, then non-great weapons aren’t worth your time. The Hexblade patron goes a long way towards solving the biggest problem here, multiple-ability dependency, but does nothing about the difficulty leveraging feats, and in any event the Hexblade may not be available in every campaign.
So this is my question: what is the best approach to getting the most from the ability to use any weapon I want? Ideally, a build that switches between weapons on the fly for different situations. Importantly, I want a character that has a reason for using so many weapons—if having just one weapon, or just relying on eldritch blast, is strictly-superior to a given approach, that isn’t an answer to the question—it’s a claim that the build simply is not supported by the system at all. Which may well be true, but be prepared to back that claim up.
Crucially, how having multiple weapons is advantageous is up to you: if eldritch blast cannot be beat for damage, for example, then a build that uses weapons for utility somehow would be great, where a build that goes for damage and just ends up worse than eldritch blast would not. But since I am not an expert in 5e, and don’t know the answer to my own question, I am explicitly looking for answerer’s expertise and judgment in how to best leverage this feature. I have offered my expertise and judgment on similar questions for D&D 3.5e many, many times, so I know this is a thing people are capable of doing.
Feats are allowed, and so is a limited amount of judicious multiclassing—but answers with less multiclassing are better. Ideally an answer considers a build’s progression from 1st to 20th, but an answer that focuses on a somewhat narrower range—explaining why it doesn’t work before that range or why it fails to grow beyond that range—is acceptable. For reference, but not as a restriction, my particular character is starting at 4th level.
Please be specific about what sources you use—nothing is completely off the table, including Unearthed Arcana, but answers that use fewer sources are better. In particular, anything that’s not in Player’s Handbook should note why it’s important and what, if any, substitutes might be available from Player’s Handbook-only play.
The reason I ask for those notes is that I am joining a game with mostly new players, and while the DM seems amenable to me making light usage of supplemental materials, I very much don’t want to push it or overburden him, or outshine my fellow players. Nonetheless, I worry that without the Hexblade, there just isn’t really a good way to do this. So I want to know what the options are, so I can make my own judgment about how much is worth asking for.
Best Answer
Hand your friend a +1 weapon
I do not believe there is a compelling optimization reason (ignoring character flavor) to regularly use different weapons. In combat, an action is simply too important to waste. It gets worse if you want to switch from sword and board to two-handed - you need an action to doff your shield and a separate action to summon your new weapon. Out of combat, weapon switching is typically either mechanically suboptimal (using a one-handed weapon when you have the Great Weapon Master feat) or pointless (switching from a longsword to a warhammer). Rarely will enemies be resistant or vulnerable to a specific mundane damage type.
That said, you know who uses a variety of weapons - your party. What if you made them more awesome?
Some math
Unless otherwise noted, the following builds assume a level 5 PC with a 16 attack stat; any feats are taken at level 4. Damage is calculated vs an AC 16 enemy. In my experience, the bonus action attack from Great Weapon Master triggers on 1/4 to 1/3 of rounds. This damage is not included in the single round calculations.
Let's look at the combined damage of a Great Weapon Master barbarian and one of two different warlocks. The barbarian has a +1 weapon from the Eldritch Blast warlock, but not from the Polearm Master one. Damage is averaged over 3 rounds (the bulk of a fight). I assume GWM's bonus attack triggers on the third round. I also assume that the melee warlock uses Eldritch Smite on the third round.
First, versus an enemy without resistances:
GWM+PAM:
GWM+EB:
Without resistances, the Polearm Master warlock's extra DPS makes up for the barbarian's damage loss from not having a +1 to hit. How would they fare against an enemy with resistance to nonmagical weapons?
GWM+PAM with resist:
GWM+EB with resist:
The minor decrease in warlock DPR is worth the major increase they can provide to the barbarian.
So is it any good?
To get the most out of giving away your pact weapon, you really should focus on Eldritch Blast instead of melee, at least in the early-mid levels.
The pros
The cons
Example build
Pretty much any Eldritch Blast-based warlock build will work here, but I will assume that you want to hedge your bets with Hexblade and convert to melee later. All spells are from the PHB unless otherwise noted.
("Drop hex? Are you crazy?" I know, I know, but consider this: PAM lets you attack with a bonus action and hex takes a bonus action to cast and to move to a new enemy. Besides, adding elemental weapon means you can still give out a magic weapon to an ally. Next level, you can give out a +2 to hit +2d4 damage weapon!)
From then on out, your build is complete - take whatever suits you when you get there. For Mystic Arcanum, forcecage (no-save hard control), glibness (basically never fail a CHA check, including counterspell checks), and foresight (be an unstoppable melee machine) are great picks. If you delay (or decide against) converting to melee, just keep hex and don't swap to the other melee invocations.
Conclusion
Overall, this build probably starts slightly worse than a pure melee warlock or a pure EB warlock; however, you shouldn't ever feel behind the curve in a group of new players. After considering the options, I believe that handing off your pact weapon is the most effective way to actually use your ability to summon different weapons.
A footnote on things that don't work:
The designers (either by accident or intent) prevented most of the invocations from applying to someone else wielding your pact weapon. Lifedrinker, Thirsting Blade, and Eldritch Smite work when you do something with your pact weapon. Improved Pact Weapon is the only one that affects the weapon itself and thus works.
A footnote on Dual Wielding vs Polearm Master:
Without the fighting style Two Weapon Fighting (from dipping a level into fighter, for example), you can't add your ability modifier to the off-hand attack while dual wielding. (Polearm Master's bonus action attack doesn't have this limitation.) A Hexblade can use CHA for both their pact weapon and another weapon, but only your pact weapon will benefit from Improved Pact Weapon (gaining a +1 bonus) and, more importantly, Lifedrinker (gaining +CHA damage on each hit).
Feats are taken at level 8. Calculated vs an AC 16 enemy. I am ignoring Hexblade's Curse, since it affects both styles equally.
(I used this AnyDice script for these calculations. The Summary tab is the best way to look at the data.)
I had to compare these at Level 9 to give the Fighter dip a chance to catch up. Even with a dip into fighter (delaying your spell progression), the Polearm Master wins out. Without it, even quarterstaff + shield wins. If you also find a +1 weapon for your off-hand, dual wielding barely breaks even (but does have a +1 to AC). The situation is worse once Lifedrinker comes into play at Warlock 12.
Finally, none of this takes into account Polearm Master's reaction attack when someone enters your reach.