As far as I am aware, there is no way to give a creature the incapacitated condition directly. However, there are 4 conditions that give a creature the incapacitated condition as part of their effects. These are:
- Paralysed
- Petrified
- Stunned
- Unconscious
There are a number of ways to apply these conditions without damaging your beloved pet. Within the Warlock spell list, the ones I could find are:
- Hypnotic Pattern
- Hold Monster
- Eyebite
- Flesh to Stone
- Power Word Stun
- Imprisonment
However, only Flesh to Stone, Imprisonment and Power Word Stun last longer than 1 minute. Flesh to Stone is severely not recommended, since you really shouldn't be able to harvest posion from a statue. Imprisonment is expensive, but will work well for this task. Power Word Stun is probably the best option, since your creature can choose to fail its saving throws till you're finished.
On the other hand, if you have an allied Bard, Cleric, or Wizard, the 3rd-level spell Feign Death suits your needs perfectly. It will give a willing creature the incapacitated condition for an hour. A Cleric wouldn't even need to waste a spell learning it, since Clerics know all their spells automatically. Since it's a ritual, he wouldn't even need to waste a spell slot on casting it! All you have to do is talk him into preparing it.
Of course, all this is moot if your DM is willing to allow you to extract poison simply on the basis that your wyvern is tame and willing to allow you to do what you want to it. Given that he's giving you a CR 6 creature for a mount, a little bit of poison is really not a big deal.
Yes.
There's nothing about the Shapechanger subtype that prevents them from being vulnerable to Hold Person, Charm Person, Dominate Person, or any other Compulsion based Mind-affecting spells.
You may be confusing Shapechanger's ability to override Polymorph effects with their own transformations with mindless creatures and their immunity to Mind-affecting spells.
Natural Lycanthropes don't have immunity to Mind-affecting spells either, so the answer is the same for either of them. Unless the creature already possesses a subtype that makes them immune to Mind-affecting spells, Charm Person, Hold person, etc. work as normal.
Best Answer
Well, Complete Mage has Fey Presence:
Obvious problems include weird fey influence on your devil, and alignment issues. Both things I’m inclined to hand-wave (for players as well as NPCs), but they are there. Also requires Fey Heritage, which is basically garbage (+3 Will vs. Enchantments). I’ll keep looking.
Aberrant Dragonmark (Player’s Guide to Eberron) works for certain (Humanoid) races but not for an Osyluth (or much of anything else that qualifies for Fiend of Corruption, though I suppose a Half-fiend Human/Dwarf/Elf/Halfling/Orc could do it).
I’m pretty sure that’s it for feats.
For templates, Half-fey (Fiend Folio, same as Fiend of Corruption in the first place) is CR +1, gives charm person at will, and a bunch of other stuff that’s pretty useful (like flight). Of course, if you’re going for something that’ll increase CR, Beguiler (Player’s Handbook II), Dragonfire Adept (Dragon Magic), Sorcerer, Warlock (Complete Arcane), and Wizard all work, and I think Binder (Tome of Magic) does too.