Melciah:
Ability requirements: None.
At the Abyss go into Raz's clan territory and keep going.
Kain:
Ability requirements: Phase.
Just before the Abyss there should be the front of a Castle, you can't
open the doors, you gotta phase through a mesh/gate looking thing.
Zephon:
Ability requirements: Soul Reaver & Phase.
At that place that looks like Stone Henge look around for a gate, phase
through it.
Turel / Morlock / Tomb Guardian:
Ability requirements: Soul Reaver, Phase & Climb.
In Kain's Castle, where you can talk to Ariel, look around for a climbable
wall then climb it. Walk around until you come to a hallway, go through
it & at the junction it doesn't matter which door you choose, go through
it & keep on going. Fall down to the ground, kill a few vamps and go
through a little tunnel, side-step and go through the front gate at the
big building's entrance. Keep going.
Rahab:
Ability requirements: Soul Reaver, Phase, Climb & Projectile.
Just after you kill Turel (See above) & get the Projectile ability,
use the Projectile to blast back a square block in the wall. There is also
a rectangular hole in the wall, that's the exit out of there. Go through
the square hole in the wall and keep going.
Dumah:
Ability requirements: Soul Reaver, Phase, Climb, Projectile &
Swim.
At the Abyss jump over it and try to jump over a broken bridge (you
probably can't though!) if you land in the water, and you probably will,
in the water do a crouching jump to burst upwards and climb up the ledge
and keep going.
Kain (Final encounter):
Ability requirements: Soul Reaver, Phase, Climb, Projectile,
Swim & Constrict.
Outside of Dumah's castle (See above) explore until you reach a dial
on the ground, constrict it until it opens a door. Go through that door
and keep going.