[4e] Monster creation

[Saintjebus]

I'm making a villain for a level 15-17 party. I'm making it shadow based(cleric of Mask) and I've gotten most of it done. I'd like to run it by the playground, though, to see if I can improve it. Thanks in advance!
High Priest of Mask
Bodak Skulk(base creature... just changing it to solo and refluffing)
Medium shadow humanoid
Level 16 solo lurker
Init: +16
Senses: Perception +10, darkvision
Agonizing gaze(Fear, Gaze, Necrotic): aura 10; a creature in the aura that makes a melee or a ranged attack against the high priest takes 5 necrotic damage before attack roll and takes a -2 penalty to attack roll.
HP:620 Bloodied: 310
AC: 31; Fort: 31; Ref: 27; Will: 31
Immune: Disease, poison; Resist: 15 Necrotic
Speed: 6

Shadow Mace of Mask, standard, at-will, necrotic
+21 vs AC; 1d8 +5 plus 2d6 necrotic, target is weakened unitl the end of the high priest's next turn.

Death Gaze, standard, at-will, gaze, necrotic
Ranged 10; targets a living creature; +19 vs Fort; if targets is weakened, it is reduced to 0 hp, if not, 1d6 +6 necrotic damage, and loses one healing surge

Spectral Form, standard, at-will
High priest turns invisible, and gains insubstantial and phasing qualities. It can do nothing but move in it's spectral form, and can return to normal form as a free action.

[Yakk]

So this is a solo lurker. A solo lurker should attack, then fade away, then attack from some other side, or something like that, right?

The "reduce to 0 HP" mechanism means that the optimal way for this NPC to attack is to club, then gaze the same target. That ... is sort of sucky for the lurker feel. Plus, with action points (as a solo), it can insta-kill party members, which is also pretty sucky for the player.

A shadow priest, with a mace and a gaze attack...

Some thoughts:
Spawning of Minions that look just like the Lurker.

Darkness zone powers, making the party move (minor action, recharge 56, burst 1 within 10 squares). Maybe concealment after 1 to 2 squares, superior concealment beyond that, and 5 necrotic damage to anyone who ends their turn in the area?

Shadow-based movement powers maybe.

An attack that splits the party up... Maybe one that teleports a party member to a nearby area of shadow, 10 ongoing necrotic damage and immobilizes her (save ends).

A gaze close burst 20 gaze attack, recharge 6 (and when bloodied) that stuns immobalized targets (save ends) for the "oh crap, that sucks" gaze attack.

Suck Life: Gain 10 temporary HP when he hits a target with a NECROTIC damage power.

Spawn shadow minions: recharge 56, standard action, close burst 10. Summons 4 shadow minions that look just like him (they must be summoned within a region of shadow), and teleport to a shadow location. Can have no more than 4 minions active.

Shadow Walk: move action, recharge 456. Teleport from one region of shadow to another with 10 range.

[Asbestos]

Personally I would just slap templates onto the Bodak Skulk and then double check the hp and defenses to make sure they add up right for a solo (they often don't, or at least the hp doesn't, when you double up on templates)

Templates that might be best.
Cleric
Death Master
Death Knight
Shadowborn Stalker

Personally, I think that Cleric might be getting too complex since it would now have cleric powers. Death Master + Death Knight would make this thing mean, especially if it had a couple of normal undead with it (yes, I know its a solo, but despite the name solos don't have to be the only monster in the fight)

[Saintjebus]

I'm calling it a cleric, but I didn't really want to get into cleric powers... I'm not real big on them as npc powers. I don't have the DMG on me, so I can't remember what the death knight and death master templates do...