The Goblin guard is a monster encountered during the Observatory Quest. He guards the gated area in which the lens mould is found. The keep key must be obtained from a chest in the dungeon to enter the gate behind that he stands in front. Players may not pass through him on the same tile, meaning that to open the gate, he must either be killed and not yet spawned, or he must be engaged in combat. It is possible to engage him in combat for the required 3 rounds, retreat, and then open the gate before he attacks again.
This article on a monster has an associated dialogue page.