Trueforms are monstrous creatures with dark souls inside of them and they reside in the Asylum . They are the strongest enemies in the game and are dangerous both up close and from a distance. They are difficult to destroy because they are very agile and have high defense. They are equipped with homing in rockets on their wrists that fire a stream of Dark Soul energy at Shadowman. Each projectile hit causes 300 damage. Due to their size, Trueforms run around quickly and are hard targets to aim at. They also scratch or stomp on ShadowMan (causing 1500 damage) if he gets too close. Shooting them with the Shadowgun charged up and using the ensigne (shield) is one way to defeat them. You can also use the Violator if you have it in combination with the Shadowgun to quickly lower its health.
Legion wanted Jack the Ripper's help to construct the Asylum and include all the “unique and misunderstood individuals” to make an “army of the damned.” The army being the Trueforms to cross the veil to Liveside and bring about Apocalypse that Mama Nettie warns you about in the intro cutscene. The first Trueform seen in the game is in the Asylum Playrooms after ShadowMan let it out of its cage prison, but he won't be able to fight it until later in the game (when he obtains the Marcher Gads.) Also dead Trueforms can be seen in the Asylum: Playrooms operating table rooms that are filled with the surgeons. The Trueforms are near a machine with a Dark Soul to be put inside of them to reanimate it.
The five serial killers each contain one Dark Soul inside them also which makes them insane much like the Trueforms, they even look like them briefly as they are being shot. They assume the Trueform look once you've lowered their health all the way down before dealing the final blow with the Shadowgun which is the only way to kill them.