Here's what was cool:
I got the basic idea from a monster that was already in the Monster Builder, but I tweaked it just a little bit, as usual. I described the monster as a giant sac of amniotic fluid filled with clearly moving THINGS, all trying to claw their way out, hands, feet, and faces clearly pushing hard against the lining. When it spawns, it convulses, and a full-grown goblin mutant claws its way out.
It freaked the PCs out when they discovered that it could move. It worked exactly as I'd hoped, using nearly every single one of its powers and getting to munch on a PC for a while.
It had support and it's support was cool:
2 of these guys. Maddening Whispers was an awesome way to hurt the PCs and throw them into the Spawning Mass. That said, as an at-will targeting Will, it was really hurting my PCs. I decided that the two Circles of Power on the floor, next to the portal they were trying to open, actually activated these guy's ability to use maddening whispers, and that the PCs, if they were to stand in these circles, could use this power too. If any of the PCs actually tried to use this power, I would have also given them all sorts of nightmares and plot related to having their minds touched by Elder Evils they aught not have communed with. Istav nearly did, but it never seemed advantageous to do so.
In addition, I decided that the circles of power would be blood-activated healing circles, so that whenever a creature adjacent to the circle takes damage, any creature within the circle gains 5 temporary hit points. The PCs definitely took as much advantage of this feature as the monsters did.
There were also other monsters in this fight. They were mostly fodder.
I described the Mutants as goblins with semi-transparent skin that was practically gelatinous that they could easily scrape piece off and throw it at their enemies. Also, to use reintegrate, they would scrape some of their flesh off, and then eat it. Finally, their bones had grown through the tips of their fingers, and then were sharpened into points. The PCs had encountered them before this. The Goblin Slaves were just normal goblins, enslaved and waiting to be fed to the Spawning Mass so they could come out as something...better.