ToME: the Tales of Maj'Eyal

I found a bloated horror in a vault, which I escaped from, and when I came back, it was no longer there. However, it continues to attack me, casting all of its spells as though it were there. It also casts Phase Door, but it fizzles every time because it's a vault. I know where exactly where it theoretically should be, based on where it can hit me, as well as the graphical effect from the fizzled Phase Doors. However, I cannot hit it. It's not invisible or stealthed, and I'm not even doing zero damage. There are no damage numbers. Right-clicking on the tile does not bring up an Inspect Creature option.