RTF is a global modification that replaces all towns in the game.

As far as I know, Heroes 5 is not entirely canonical compared to 6 and 7, as the Ashan universe only appeared in them. Therefore, the neutral Death Knight has no reflection in the canon. It's a completely different matter that the class of Necropolis hero-powerhouses is also called Death Knights. Hence, the conclusion is that Death Knights are just a type of necromancer.

At the beginning of the events in Heroes 5, there was no Necromancer state at all - they were all driven into the valley, from where Markel escaped and arranged a total revival of the Necropolis - it is quite logical that the Death Knights are the remnants of the Necromancer heroes from the sixth game, and they disdained to join Markel (after all, he was Sandro's student!), but they joined the pious Arantir without hesitation.
At the time of the mod's events, the vampire knights created a base in Erathia; most of the recruited knights are captains of the Black Guard mercenaries, but after Arantir's death, their loyalty wavered, and Death Knights appeared, similar to those in Heroes 6 - vampires who have not reached the super level after which the body disappears, but are already ready to crush everyone.

Added 4 minutes ago

Can the draining ability be made to restore health as a percentage of the killed unit? That is, there is a stack of 100 peasants, a knight killed 70 with the first blow, but did not heal, and when he finished off the rest, he healed as a percentage of the remaining 30.

Added 1 minute ago
By the way, about the heroes. What will the hero models be? Only the amount of health restored can be changed in the draining ability