Dark Archons are made when two Dark Templars warp into one. They are a purely spell-casting unit and have no attack.
During their long exile, the Dark Templars have faced many challenges. Struggling and adapting their arsenal of skills and converting their biology to suit their harsh environment has ultimately resulted in the Dark Archon.
Like their High Templar brethren, two Dark Templars sacrifice themselves in a Psionic melding giving rise to the Dark Archon. The powers of these avatars of Psionic might extend well beyond those of the regular Archon, more than compensating for their lack of any physical attack capability:
The burning, crimson effigy of the Dark Archon spreads fear across any battlefield. These powerful creatures of living Psionic energy embody the eternal wrath of the Dark Templar. Created by the merging of any two Dark Templar warriors, the Dark Archon can wield the mysterious energies of the void. These dire, volatile beings are so powerful that the Dark Templar have outlawed their creation for a thousand years. After the Zerg Swarm destroyed the planet Aiur, the Dark Templar have realized the necessity of their greatest weapons.