From Terraria: Otherworld Wiki
A brightened image of the Crimother as it appears in the Swing Grappling trailer.
The Crimother is a boss seen close to the end (0:44 - 0:47) of the Swing Grappling trailer. It seems to be based around the Brain of Cthulhu from the original Terraria. The spikes surrounding it seems to jut out and in, presumably to be used to attack the player.