I'm terrible at formatting so sorry if it looks all text wally... Couldn't figure out how to make the font that separates the sections any bigger.

Part 1, Bomb changes

Warmaster's Charge - Seerus ignores all unit collision. If Seerus runs into a player while charging that person will be damaged, knocked away, and stunned briefly. (3 seconds)

Dark Retribution (Enhanced) - Same as his bombs in the normal fight, but with a slightly larger blast radius. They also have a chance to briefly stun the player (3 seconds) if they're hit by it.

Grip of Corruption - The shadow version of Seerus' arena does not start with a swarm seed in the middle. Instead, every second time he charges across the arena Seerus drops 3 "corrupted" bombs on the ground that leave swarm seed zones on the ground after their fuse ends. They function the same as seeds in normal shadow lairs but they're destructible if you manage to suppress them down to their source. The zones do not overlap and cover 30-40% of the arena if left alone.

Part 2, Hammer changes

Crushing Corruption (Hammer Smash) - Same as his hammer attack in the normal fight, but whenever he hits the ground he creates a shockwave (4-5 tiles) that deals a small amount of damage stuns anyone hit by it. (5 seconds)

Hammer Blast - Same as his blasts in the normal fight, but with a slightly larger blast radius. They also have a chance to briefly stun the player (3 seconds) if they're hit by it. (Deja Vu)

Corruption... Blast? - Seerus' final attack before knocking himself down is an ultra enhanced version of his Hammer Blast explosions. Instead of simply doing a plus pattern the explosions from this attack will travel along the walls and then back to Seerus himself, dealing increasing damage (and possibly growing in size) as it travels. This attack is always preformed from the middle of the arena.

Here's a picture so it's a bit easier to visualize. Imgur Link

Spin Attack (Couldn't think of a name) - Same as his phase ending spin in normal, but with a larger reach. Has a chance to stun (5 seconds) or inflict a strong burn to anyone hit by it.

Part 3, Battle Pods & Gremlin Support changes

Battlepods - In the shadow version of the fight, Seerus' battle pods have all been upgraded to have the ability to fire rockets that inflict different status effects. (Left = Storm, Middle = Freeze, Right = Fire) Additionally, the players must defeat all three battlepods before Seerus will come down to fight them. They no longer have the abilities to respawn or heal themselves but have highly increased health and shorter vulnerability windows.

Gremlin Support (New) Void Mortafires (Fire/Stun), Void Thwackers, Scorchers, Menders, etc. Mortfires and Thwackers constantly respawn. Menders and other annoying gremlin support units are spawned when a phase ends.

Part 4, Rewards

"Void fragments" used to craft new 5* items.

Corrupted Mask of Seerus -
Normal Defense/Shadow Defense
(Melee) CTR: Medium
(Melee) ASI: Low

Stun Resistance: Maximum
??? Resistance: Maximum
??? Resistance: Maximum
??? Weakness: High
??? Weakness: High

Basically a shadow defense version of the existing mask with added stun resistance (since the lair would be fire/stun themed). Unsure on what the resists and weaknesses should be. Having it be the same would make it a great FSC helmet just like it's counterpart... but swapping the statuses and weaknesses (freeze/poison resist and fire/shock weakness) would make it truly flipped.

Shadowcast Battle Cloak -
Normal/Shadow
(Melee) CTR: Medium
(Melee) ASI: Low

Stun Resistance: Maximum
??? Resistance: Maximum
??? Resistance: Maximum
??? Weakness: High
??? Weakness: High

A set of armor to match the mask. Resists and weaknesses would match whatever the masks resists are.

Warmaster's Battle Cloak -
Normal/Elemental
(Gun) CTR: Med
(Gun) ASI: Low

Fire Resistance: Maximum
Shock Resistance: Maximum
Poison Weakness: High
Freeze Weakness: High

Armor matching the armor that Seerus wears. Same bonuses as the original mask.

Darkfire Reprisal -
Elemental Damage
??? chance of causing Strong Fire

This can either be a copy of DR or a fire vortex bomb (similar to Celestial Vortex). An elemental DR makes more sense but I think having it be a vortex would make it more unique. Placing it would spawn a small void area that's draws enemies into the center before exploding into a cube-like shape.

Swarmcore/Voidspawn/??? Rocket Hammer -
Shadow damage
Uses same combo as WRH

Nothing much to say. Just a shadow version of WRH that has a different color scheme to it.

Extra -
It would also be kinda cool if there was a chance (maybe like 1 in 60) that killing the boss makes him drop a unique accessory. Things like Seerus' bomb bandoleer, or a smaller version of the rocket he carries on his back, or an aura that makes your footsteps look like small explosions.

Part 5, Requirements

- Courage to face a dungeon full darkness, fire, and annoying stuns

- Ownership of the OCH expansion

- One Shadow Lair Key