TransformsYou may occasionally hear people discussing Transforms or refering a transform "popping". There is a chunk of history of the national game there. Transforms are intended for experienced characters and players, to offer them an option for when their character fails to resurrect. Instead of going to your reward or punishment in the Graveyard realms if someone has performed the Formal Magic Transform to TYPE CREATURE then they will becomes that sort of being instead. Having passed on their character is now a non-player character under the control of the plot team and it may or may not appear at any time in the future.
Pop ModsThe amusingly named "pop mods" are a game module (adventure) where you will be facing such powerful opponents that the plot team as chosen to allow a lesser version of the powerful creature you will become upon permanent death to appear briefly on that module. They are rare events intended to let you have a moment of fun playing a truly powerful creature facing an even more powerful opponent.
Character versus Character SituationsThese transformations are not intended for use against your fellow players. David Cashel, the former General Manager, wrote "The national policy on Transforms and Curses of Transformation is that they cannot be used in a PVP scenario."
However keep in mind that if you are turned into a werewolf, say, with a plot generated Curse of Transformation(CoT), it is up to local plot if you can attack other players -- just as any other NPC, and your fellow players should view you as dangerous opponent and may try to kill you in response. This death might or might not count against you, it will depend on the local plot team and circumstances. The methods of removing a CoT varies with the specific curse and the plot team involved.
Availability and StatsThe available forms one can transform into are limited by the scrolls required to perform the Formal magic and the stats are based on those of the underlying creature and are always entirely at the discretion of the local plot team. If one has a transform one should notify the plot team in advance and sharing the information about the transform from your home chapter so they can decide what stats they will allow it and consider if there will or won't be a pop module or a situation in a field fight that might 'pop" a transform.
Curse of TransformationThe standard fantasy trope of someone bitten by a vampire rising as one when they die is well covered by the Formal Transform. But the werewolf's bite and similar periodic infects are better represented as a Curse of Transformation, an invisible effect which will appear under certain specific circumstances. They are challenging to handle but can sometimes be removed by a Formal spell to remove the magic from them. Plot will provide you with stats and role playing notes and the conditions under which you transform.