Slamming his own head into the ground to stomp Isaac, firing blood tears in 8 directions.
Its attacks are identical to Daddy Long Legs, except more white slowing creep comes from his 'feet' and using his headslam attack spews 8 diagonal shots across the room.
Unlike the other alternate unlocks (Steven, C.H.A.D. and Gish), Triachnid does not have a specific item drop.
Every stomp of Eternal Triachnid leaves a small puddle of white creep and sometimes, an additional Sack. All other attacks are same as its normal counterpart.
This boss is a reference to another of Edmund McMillen's games, Tri-Achnid, which can be found here.
The story of the flash game is that Triachnid is trying to find a safe haven for his children. In Wrath of the Lamb, Triachnid presumably would have replaced Daddy Long Legs.
Before version 1.15, Triachnid would replace Isaac in the Cathedral as the final boss if Isaac was defeated while The Polaroid trinket was being held. This was confirmed as a glitch by Edmund McMillen after it was replaced with ??? with the 1.15 patch and removed from the game.
With 1.333, Triachnid was added back to the game and it correctly unlocks as an alternative boss to Daddy Long Legs instead.
Menu screen after defeating Triachnid prior to 1.15.
The secret unlocked for completing the Womb multiple times.
A Triachnid battle in The Womb.
Isaac vs. Triachnid (Daddy Long Legs II) - The Binding of Isaac Wrath of the Lamb