It's a Growing Champion-Golem.
As you can see, the current accuracy/evasion/damage boost of this Golem is 326%, and the current damage reduction is 76%. This means that this Golem is too strong that... God, please save me from this monster.
I've sometimes wondered what the game would be like if monsters were more intelligent. Like, if burning monsters sought water, or if dwarf wizards kept using magic at point range. Now, imagine that thing being intelligent. It'd probably wait near the stairs to make sure you don't escape, and also growing until you reach it. And if it hasn't grown enough, it could trap you in a circle with it teleporting away, then teleporting you to it then teleport away again, so you can neither hit it nor than once in a row run from it and it gets extra turns to grow.