Guard no. 21 is one of two warped terrorbirds who guard an important door inside the Poison Waste Slayer Dungeon, the other being Guard no. 72. The two are only encountered during The Path of Glouphrie quest, appearing talking to each other in a cutscene which ultimately ends with them pressing a red button in panic and releasing a toxic gas that almost kills the player.
Both of the guards can speak the common tongue of Gielinor, though most animals such as terrorbirds cannot. It may be, however, that Glouphrie's magic turns the terrorbirds intelligent when it warps them and makes them invulnerable to most forms of attack.
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