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Transformation is the art of permanently becoming something else, or of making someone else something else. Both Death and Nature Mages can do both of these, but the transformations of death are far more permanent. Transformation resets the Age of the subject, and it may heal afflictions.
Only those with living, physical bodies may be transformed. This excludes the Undead (and Demons), the Inanimate, Spiritforms, and Illusions.
There are two Death rituals that transform the user. Twiceborn and Lichcraft. Neither can be used by Plants, nor by Pretender Gods or Prophets. The resulting form is Undead, so the user cannot transform again afterwards. Both of these spells have their own pages.
The only Nature ritual for transformation is Transformation. This one has a random result, though the result is almost always living.
How the spell changed in the transition from Dominions 5 to Dominions 6 is currently unknown. The Dominions 5 version of this illwiki page gives much more detailed information on possible results. However, it should be noted that the spell's description was misleading in Dominions 5, and that this doesn't seem to have changed in Dominions 6.
To reiterate, the above is how the spell works in Dominions 5. It is not clear if Dominions 6 changed how Transformation works, however the spell's description did not change. The list of forms that one can Transform into, good and bad, does not appear in the Dominions 6 Mod Inspector, and it would be useful to investigate whether Dominions 6 adds new potential resulting forms or not.
One known difference between Dominions 5 and Dominions 6 is that Pretender Gods and monsters with Polymorph Immunity are now unable to cast Transformation, just as they are unable to cast Twiceborn and Lichcraft.
Two items, the Lycanthropos' Amulet and the Pebble Skin Suit, will eventually transform their wearers. The former has a 2% chance to make you a Werewolf each turn, while the latter has a 2% chance to make you a Troll. Getting either of these may happen involuntarily, since they're cursed items, so this might not happen willingly.
There is one Death ritual that transforms others, the Ritual of Rebirth, which turns a fella into a Mummy. It does work on the Undead, unlike other Transformations; however, the subject has to be already dead. You may also use this on someone more than once to resurrect them again, making it the only Transformation spell that also (technically) works on the Inanimate; more details are divulged below.
There are two combat spells that transform others: these are Curse of the Frog Prince and Polymorph. The former makes someone a Frog (one of those Mindless bad results mentioned under Transformation above), while the latter turns many people into Boars. It is unclear if making someone a Frog is as bad for them as becoming a Frog via Transformation.
Resummoning a specific unit (i.e. Recalling a Pretender God) transforms them back into their original form. The only exception is the Ritual of Rebirth, which summons the fellow back specifically as a Mummy.