From Populous Wiki

Obelisks are magical objects that can only worshipped by a shaman. When worshipped, the two sections will spin 90° and give the worshiper the reward or effect. If the shaman worshiping stops or is killed while the sections are rotating, the obelisk will give the reward to the blue team.

Unlike Stone Heads, Gargoyles or Totem Poles, there is no typical obelisk reward - it can be anything from a change to the land to a high-level spell

Stone Head • Totem Pole • Obelisk • Gargoyle • Vault of Knowledge