It's a tower-defence event you start yourself. The Tavernkeep moves in once you've beaten the Eater of Worlds or Brain of Cthulhu - buy the Eternia Crystal Stand and an Eternia Crystal from him, place the stand, put the crystal in, and waves start walking at it from both sides. If they break the crystal you fail.
There are three tiers. Tier 1 is what you get early, tier 2 opens after any mechanical boss, tier 3 after Golem, and each tier adds tougher waves plus a boss (Dark Mage, then Ogre, then Betsy at the top). Kills drop Defender Medals, which is the Tavernkeep's currency for the sentry weapons and armour.
The mistake most people make is fighting it like a normal invasion. Build a flat arena around the crystal and spend your first medals on sentries, because you can't be on both sides at once.
