In StarCraft, the Protoss unit created by merging two High Templars is the Archon.
Archons are powerful psionic entities formed when two High Templars combine their energy. The merging process consumes the original templars and creates a single unit with very high shields and devastating attacks against both ground and air targets.
The Archon is one of the clearest examples of Protoss technology blending with psionic power. It is especially useful against biological armies because of its strong area damage and robust shields, although its effectiveness depends on protecting it from focused fire and shield-draining effects.
The Dark Archon is a different unit. It is formed by merging two Dark Templars and has specialized abilities such as Mind Control and Maelstrom. The two units share a name but have different origins and battlefield roles.