I'm having trouble finding monsters in the MM that I can re-skin effectively to have powers that fit the monsters in the Asylum. The place is filled with humanoids that have been infected with a disease that is essentially sentient Lunacy.
"Sentient Lunacy" screams insubstantial monster to me. Check the ghost/specter/wraith entries for some common insubstantial monsters that could represent this entity in it's 'natural' state. Alternatively, if the monsters are semi-insubstantial, consider allowing some type of damage or attack to disable this resistance (like how Troll regeneration is disable temporarily after taking fire damage).
As for how this sentient lunacy is transmitted... consider using one of the disease rules from the DMG or maybe, going with the sentient creature angle, you could look at the possession rules for ghosts. Specifically, check the "Trap Haunt" on p.116. There's also a pretty cool spectral vampire in the DnD online compendium.
Some have phantasmal limbs, tentacles coming out of their mouths, or are simply insane mentally.
1. See above for insubstantial qualities.
2. For some examples of tentacled monsters and their attacks;
Carrion Crawler p.40
Choker p.42
Grell p.144
Mind Flayers p.188
Roper p.222
While re-skinning any one of these as is may not be *exactly* the monster you envision, they should give you some idea of how to word, balance and structure a tentacle power of your own creation.
3. As for mental insanity, I think berserker rage seems to fit the bill. I believe Orcs and Gnolls have properties and powers that reflect their bloodthirstiness.
Also, while I'm not sure which monsters have them, typically "rage" powers are activated either when the monster is bloodied, or when it targets a bloodied character. Alternatively, the monster's normal attacks could do more damage or have special properties under those same conditions.
Could anyone suggest some monsters with powers that you think can be re-skinned easily into odd psychic blasts, spectral body parts or chaotic powers?
Just about any power can be reskinned to be a psychic blast. Just change the damage type.
*A blast or burst of fire doing ongoing fire damage becomes a psyhic blast or burst doing ongoing psychic damage.
*An attack that deals knockdown becomes a psychic assault that does knockdown.
*A web attack that immobilizes characters becomes a psychic compulsion/fear effect that immobilizes.
And so on...
As for those so called chaos powers... check the MM entry on Slaads on p.237. They have a lot of really interesting powers with unique effects. They might inspire some 'chaotic' powers of your own.
While I can't just point to a monster that can be re-skinned wholesale for your game, hopefully this gives you some good ideas on making your own monsters.
For easy stat block calculations, try
Asmor's Math Cruncher. Good luck!