They are exceedingly dangerous Shikadi. They can teleport themselves to any place they wish, and also shoot deadly blasts of energy (called "electrospheres") along the ground. It should be noted that the electrospheres can be stopped by shooting them. Shikadi Masters only appear guarding the Quantum Explosion Dynamo. They can only be found on normal and hard difficulties.
The official help says:
See also List of Keen 5 Creatures.
Appearance in Keen GBC
The Shikadi Masters return in Keen GBC where they team up with the Droidiccus and the Bloogs to try and destroy the universe, again at Mortimer's behest. Here they are more shadowy and solid, being bright blue and ghostly, rather like the 'shadow beings' they were described as in Keen 4. They behave almost identically, though they are smaller than their normal Shikadi counterparts. Keen meets them on their possible home world Shikadi World.
The manual uses the same description as found in Keen 5.