What is tournament of the damned?

I heard it's some skulltag episode, but it does not appear on the episode select. Can somebody tell me what it is?

It was planned to be a single-player skulltag (the gamemode) campaign.

It was never actually implemented because the Skulltag bots only know about deathmatch and are absolutely unable to do anything else (they don't capture flags, they don't grab skulls, they don't hold domination points, and they don't attack monsters). So a single-player skulltag campaign would have been pointless.

Instead it was replaced by a single-player invasion campaign.