Grubclaw was a rook in General Ironbeak's army. He and Ragwing were keeping watch when Cornflower, disguised as the spirit of Martin the Warrior, appeared to scare them. Grubclaw spread stories amongst the rooks and unwittingly helped the Redwallers because Ironbeak's minions no longer had the morale to fight. He was thrown into the Abbey pond by Constance; it is unknown if this action killed him, as his final reference can be interpreted as being either soaking wet or dead. He no longer appears as a character, either.
There's a typo in the paperback version of the book calling him "Grubwing." In actuality, Brian Jacques was referring to "Grubclaw."