Austin grins and steps in front of the group, picks out a point and launches a fireball into the room (I put a red circle for the AoE, but that goes away immediately. Its just there for you guys to see.
Seeing Austin moving forward though his own eyes, Mourne sees where the sorcerer is focusing and suddenly the easternmost 'copy' of Mourne disappears. "Oh this is..." wheeze "...good. You already..." More wheezing "...made me jump." Mourne says, which is barely audible over the sound of Austin's exploding fireball.
The four horrors were not ready for Austin, turning to look as he begins the spell and are enveloped in flame. Chunks of rotting flesh fall from the smallest of the creatures in the room into flaming piles of goo, possibly reducing the damage of the flame.
Terrain: The room is filled with thick viscera, slippery especially considering the softness of the ground. Whenever you move, you must make a DC 10 +1 per five foot space moved DEX Save or slip and fall prone. How much you fail determines in which space you fall.
Sadly, each of your enemies has feet with claws that grip into the terrain, nullifying this effect for them.
Eithne is currently flying and unaffected.
Dredge
Dex Save: -1, Initiative: +2
Wretch
Dex Save: +2, Initiative: +3
Plague Fiend
Dex Save: +3, Initiative: +5
Special Abilities
Plague Rot: On any damage taken, roll 1d6. On a 1 or 2, the damage is halved due to chunks of rotten 'meat' falling off of the Plague Fiend where it was hurt.
Mourne
Dex Save: +0, Initiative: +3
Special Abilities
Allies: Has the ability to make multiple enemies into allies in a 10 foot radius. Enemies get a DC 15 WIS save or believe Mourne to be their Ally, to the point of attacking their own true allies. If Mourne uses this ability on a character a second time, they gain advantage on the save. After that, the ability can no longer affect the target for a week.
Pre-sense: If Mourne makes an Ally of someone, he can sense through their sense while he concentrates on that ally (or former ally if the Allies ability has been broken) for up to 1 hour.
Jump: Mourne has the ability to 'jump' to one of his Mirror Images (if the spell is active), destroying that Image. Mourne can do this as a Reaction.
Multiple Concentration: Mourne has the ability to concentrate on at least 2 (possibly more, we don't know yet) spells/abilities at once.
Summary
INITIATIVE!