Hulk is a modification for Quake 3, pitting normal players with normal weapons against the Hulk, an enhanced player with immensely powerful weapons. The Hulk is randomly selected when players join a server, and killing him causes another player to become the Hulk. Frags are gained by doing damage to the Hulk, not killing him. As the Hulk, killing players gives you frags normally.
The Hulk's power increases based on the number of players on the server, but staying alive as the Hulk for an extremely long time is not easy - players start becoming Hulk Killers after a minute has passed without the Hulk dying. They gain Quad, and the Hulk cannot tell who is a Hulk Killer.