could demons (or succubi/incubi) shapeshift??
Demons are a type of monster, and unlike peoples, monsters can't learn/cast spells that they weren't already born with. For example, all drau are born with innate powers of necromancy. All adhene are born with elemental hazard spells. All pixies are born with perception spells.
Demons only shapeshift once in their lives, when they transform from their child disguise into their adult demon form. They have no control over this spell, it's more like an effect that happens to them. Once this happens, their innate curse abilities are activated.
As adults, demon can bestow curses onto peoples, and exactly what kind of curse it is depends on the demon. Usually these curses involve some type of bodily harm (warts, itchiness, body part shrinkage or deformation, etc.)
Demons can't learn transmutation spells, so they can't shapeshift themselves. However, another mage can cast a transmutation spell upon them and they can shapeshift that way.
Demons do not have souls and transforming them into a human, for example, will not give them one. But they might look totally indistinguishable from a human, or whatever they were transformed into. Transforming them might also take away their curse ability as long as they are in human form.
The only monsters who can shapeshift are, strangely enough, kobolds! But the thing is, kobolds are so genetically messed up that not all of them can even cast their innate spells. Oftentimes their IQs are too low or their bodies aren't healthy enough to shapeshift without botching the spell and killing themselves. Or the spell is poor and their new form just looks...bad. Or they can only transform themselves for a few seconds or minutes at a time.
Kobolds in the Seelie Court are known to transform themselves into animals and beg peoples for food, because in this land, animals are treated much more humanely than kobolds. They might also disguise themselves as beautiful fae or scarier monsters to get what they want.
All kobolds have shapeshifting potential, but few of them can do it successfully. I think if the Seelie queen, Titania, invested in the poor kobolds instead of oppressing them and trying to sweep them under the rug, many more of them would realize their special shapeshifting talents.
In the series, a male kobold named Tojum (she/her) was determined to transform into a beautiful female. Tojum didn't really care what species, as long as she wasn't a kobold anymore, because kobolds are oppressed in her culture. She stole a magical amulet of shapeshifting from the local arcane college to help her achieve this, because she didn't trust her innate powers to pull it off successfully. So, like demons, kobolds can also shapeshift with help from other mages or magical objects.
