Choose a fun EL 12 evil Monster to play

[Shadowbite]

Going to be playing a one shot where everyone is evil and playing monsters. Any suggestions on what to play? We are allowed up to EL 12.

[Zweisteine]

Play a pixie, and stick on a few class levels!

Arrow Demon would be cool, but their ECL is technically 18, though they're only CR 7.

[Urpriest]

Succubus is a great ECL 12 evil monster. Lots of abilities, though you'll be more of a utility character than a combat character.

Edit: Actually, if you literally mean EL 12 there are more powerful options...assuming you meant ECL though.

[Palanan]

The Essence Reaver, from Secrets of Sarlona, is one of my favorite creatures. Drink spells and psionic powers from your targets, and then use them yourself? Yes, please.

It's officially CR 9, I think, but could probably be rated a little higher, and/or stack a couple class levels on. I threw one at an experienced party in a wilderness campaign and it completely wiped the mat with them.

[(Un)Inspired]

Is level adjustment buy off in play? If it is I say Drow Warblade/ eternal blade

[Hurnn]

CR 12 or ECL 12? ECL 12 go succubus, you are amazing at that ecl. CR 12 Mature adult White dragon, you would be prety beastly in combat, 3rd lvl sorc spells, a couple decent su powers, flight, crazy hp and ac and good stats.

[Shadowbite]

Okay seems the DM was confused. Total HD = 12. Any interesting choices?

[Fouredged Sword]

12 HD, no LA. Take something with high LA and low enough HD to cram in great class levels.

Succubus. Go succubus / Ur-priest X until you get 12 HD. 6 levels of Ur-priest means 6th level spells.

Young Red Dragon Sorcerer 1 could also be alright.

[Manly Man]

War Trolls are great right out of the box as melee bruisers, and they can be decent with a bow too.

[Biffoniacus_Furiou]

Ghaele (http://www.d20srd.org/srd/monsters/ghaele.htm) with the Corrupt Creature template from BoVD and two levels of Cleric.
Corrupt Creature can be applied to anything except an outsider.... I'll get back to you on this one.

Maybe an Immoth or Spell Weaver from MM2, and add on one level in each of Mindbender and Paragnostic Apostle.

Consider a Juvenile Phaerimm (http://www.wizards.com/default.asp?x=dnd/we/20040313a) (at the end of the .pdf) with eight levels of Sorcerer + PrCs, which at 12 HD would be able to cast spells as a Sorcerer 20.

Possibly an Ulitharid from Lords of Madness, it's a really big Mind Flayer. Definitely go with the psionic manifesting version in the sidebar.