I had the idea of a mad mage filling up a dungeon with monsters and traps, and then I figured there whould need to be reason for the mage to be mad. A Lovecraftian horror locked up by the mad mage would work as an excellent motivation for the mad mage so I decided I should go with that.
So now I have to find a monster that could make a mage go mad. Any suggestions would be appreciated.
It depends on the CR of the scenario. If you're playing low level CR, typically you're looking for aberrations or outsiders as your capstone boss, and likely the story is that the wizard summoned one using Planar Binding and it escaped.
Qlippoth are a good choice for a variety of perverted, disgusting things-which-should-not-be. For example, the wizard whistled up a Shoggti, but fumbled the magic circle binding it and the horror from beyond drained the bumbling conjurer's wisdom, turning it into a mindless thrall. Like all Qlippoth, the Shoggti hates all life, and so uses his puppet wizard to wage war on the mortal plane by sending out monsters from his tower-fortress to destroy the living.
For higher CR adventures, you can't get more Lovecraftian than the Great Old Ones. Our wizard, seeking knowledge best forgotten, indulged in too many castings of Contact Other Plane, drawing the attention of those who reside beyond. They sent their emissaries to him, warping his mind beyond recognition and driving him to recreate his surroundings in a mirror of the otherworldly horrors his insane mind perceives.
|Bob Bob Bob|
Yeah, I'm a big fan of the Allip, since it opens so many wonderful possibilities as to the reasons behind the demise of the original creature.
In one scenario I created, I had a room full of small sized Allips, each of them resembling a murdered orc child that had been kidnapped from a local tribe. The PCs had earlier been told by human townsfolk nearby that the orcs were responsible for recent kidnappings, but they were smart enough to discover the orcs were innocent, and actually just as much victims as the townsfolk.
I'm not that big a fan of including suicide in my games, so the Allip is out. The great old ones are a bit too strong, though I am aiming for a mega dungeon. The Shoggoth is disappointing. There are lower CR monsters that do madness better than confused for 1d6 rounds. The Colour out of Space looks good, but I think I want to go for the Iatavos Qlippoth.
I have three reasons for this. The first is that you want to imprison the Iatavos. It's the only one of its kind, and if you kill it the abyss will simply birth another one.
The second reason is that I can adjust this unique creature as needed, as described in the fluff.
And third, adding a bunch of Nyogoths should raise the CR of the encounter and create a more interesting fight.
A Chaos Beast would be an interesting choice too, though I'd need to advance it at least a little. (Adding 5 hit dice would be enough to make it grow to large, for example.)