The Worm Boss is a large creature that will spawn on Soft Resin blocks. They can be seen defending the Overmind but may only spawn on soft resin created by a Tunnel Nuker's explosion. The acid that it spits is deadly to the player. You are able to withstand around 1.5 seconds before dying, so avoiding the worms is critical. The attack can also go through blocks, so a wall will not help to negate the spit. Coming into contact with the worm will instantly kill the player.
The Worm Boss has a very high number of hit points making them hard to kill. If a Worm Boss spawns, placing Turrets with plenty of Power with eventually kill it. Worm Bosses will despawn but only after a long period of time.
However, it is possible to kill him removing the blocks where they're rooted. Because Worm Bosses can cause so much trouble if they are near your base, clean up large patches of soft resin before problems arise.