The last engine converting pain to desire is a rough, black cube as big as a cart. Pain inflicted in its presence is said to slowly fuel a change in the inflictor. And in this way the infamous lich Red Petals was said to gain the ability to stop time twice in a day, children tortured around him as he lay reading on the cube.
Tally hit points of damage in the presence of the cube. Once they equal the hit points the damage dealer needs to go up in level, the damage dealer receives an extra spell memorization slot for that level. This can only happen once a level.
I'd say these spell slots would be usable even by non-wizards. Because that would require a lot of damage to be dealt for it to become crazy powerful.
Psionic powers is another possibility if you want to avoid spell-slots.
I might want other smaller pleasant effects such as removing addictions, removing fear and troubling afflictions of the mind, insanities and phobias and such. Some positive effects that would happen immediately and get players messing with the engine. But then, maybe the engine is more relevant as an artifact sought by the powerful of the campaign world-- something to be found and fought over in a grander way than just a toy to be examined by players in a dungeon.
Are engines just dark angels? The world is never as simple as a binary. Is it? Are engines the engines that drive the function of the angels? If so, what strange and terrible fuels could other engines run on?
Note: this came from a dream I had last night, the line "you are the last engine turning pain to desire" in my head as I woke up. There was another line from earlier in the night but I forgot it.
More engines sure to follow.