Fires a volley of "Flaming Wood" (flaming spikes) at the player. These pass through blocks.
Fires a jet of "Greek Fire" (sparkling flames). These do not pass through blocks. They are affected by gravity and remain on the ground for a while, doing damage to players that touch them.
Notes
Drop chance increases linearly with the wave it is in (lowest drop chance in wave 4, highest drop chance in wave 15). In wave 15 it will also drop a Trophy and other loot.
Mourning Wood can initially pass through blocks when it spawns within them, floating towards the player until it reaches an open area. It resembles a Wraith's behavior. Otherwise it walks on surfaces and cannot jump.
They do not despawn after killing all available players (however they do on the 3ds version).
Mourning Wood is immune to most debuffs, but is vulnerable to the Ichor debuff.
Tips
Pay attention to when a Mourning Wood stops walking. This means it's going to attack.
It is advisable to not enter a small room while fighting a Mourning Wood because its "flaming spike" attack can go through walls.
Mourning Wood's "Greek Fire" attacks cannot pass through water, therefore submerging Mourning Wood at least halfway up is an easier way to defeat him. Note the "flaming wood" attacks DO pass through water.
Trivia
Mourning is the grief over someone's death, and fits in well with the Halloween theme of the Pumpkin Moon.
The name is also a pun on "Morning Wood", a slang term referring to nocturnal erection in males during and immediately following sleep.