What is this monster and how do you get it to appear?

Question

This is an image of a monster that appeared... somehow in my game. It's not in the monster list (at least, as of a brand new game or install).

What is it and when or how does it appear?

Anthony Mastrean · Accepted Answer · 2017-01-09 20:16:26Z

After encountering one, the monster list calls it a "ghost". It says

Can move through walls.

When killed, stun a random enemy.

One of the developer's design notes says:

... I gave curses the extra effect of ghosting...

And another says

... curses got the default ghost effect...

Which seems to imply that a tile that curses a monster will eventually turn it into a ghost?

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