Overall
neat idea!
monsters that change during the course of combat always add a twist (often it happens as a two-stage monster... monster-prebloodied and monster-postbloodied, but you've basically made a three-stage monster... phase 1, phase 2-prebloodied, and phase 2-postbloodied)
Math
I'm not looking at the math for attack bonus, defenses, etc. I'm presuming that you're using some of the given formulas with possible tweaks for flavor.
Phase 2, Dark Reposite
As written, you'll 99% likely never get to use this.
Creatures almost always provoke OAs on their own turn. But you can't use immediate actions on your own turn, thus you can't respond to an OA by using an immediate action in most cases.
You can have the exact same effect by changing it to: "Free action, at will, max 1/round"
You can make it slightly better by making it: "Free action, at will"
You can simplify it by removing that altogether and instead changing Shadow Laser to add the line: "Using Shadow Laser does not provoke OAs."
Alternative way to simplify it: "When taking OAs from movement, the attacker takes X (5?) damage due to body spikes.
and so on ...
Add a burst
huge creatures don't get many benefits (the main ones being they are harder to push over a cliff and also that even burst 1s are huge), but a big disadvantage in the ease at which they are flanked.
I'd give it a burst 2 ability like a standard action encounter ability - such as a
Flail Sweep - Close burst 2. The slayer makes a flail attack against each creature in burst. On a hit (in addition to damage) the target is also knocked prone. On a miss: half damage and not prone.
This will help it when it does get flanked (which, as huge creature, will just happen easily). Or to kick it up a notch, change it from standard action; encounter to "immediate interrupt when a third enemy moves adjacent to the slayer; encounter"
add a rider effect/condition or two
even though it is a skirmisher, it is effectively taking the place of a solo creature. As such, i'd consider giving him some extra rider effects on the attacks. you do it some with the 'knocked prone' during phase 2 shadow laser. but maybe double attack also makes the target dazed (save ends) but if you do that, i'd change double attack from "shift before each attack" (i.e. shift attack shift attack) to "shift between each attack" (ie attack shift attack).
charger
this is a pure personal preference, but anytime i see a
huge warforged, i just think it needs to steamroll over pcs. so i'd be inclined to give it some sort of encounter charge-based attack and that it can move through enemy spaces during the charge, but must end in an unoccupied space, making a flail attack against each enemy whose square it passes through (and some bonus damage to the last target).
- it would still provoke OAs during this movement so be sure to have your OA deterrent (either damage to the OA attacker, or else the slayer simply doesn't provoke)
party composition?
curious: what type of PCs are there (not just talking classes, but what are they focused on). i.e. do you have lots of melee PCs? if not, then the close burst attack suggestion won't really do anything and isn't worth it. Do you have a lot of heavy hitters? then the HP on the slayer will dwindle down fast; do you have strong controllers (they block movement or daze/stun a lot)? if so, the problem with single creature fights is that they are bound to get locked down and not have much chance to act.
anyway, i speak with no real authority though, just random thoughts....
