The Diminisher

The Diminisher is Commander Zombreaker's destruction machine. It serves as a tanker and can block a 100-pound weight worth of damage, before exploding in a HUGE radius and dealing 5000 Damage to anything inside of the radius.

I wouldn't recommend luring him to the Core and giving him all you've got. It's best you keep him at one Undead Lock (again, aka, gate). If the Core is at 5000 HP or less and The Diminisher is close to dying and near the Core, say goodbye to your 2 hours worth of playing because you're going all the way back to Stage 1.

No kidding. Things like Kamikaze Guests can absolutely ruin you if something explodes with high HP.

Not much else to say, aside from the fact that he doesn't currently spawn and might get a rework in the future.