Archon

The Archon is a Protoss entity formed by the merging of a pair of High Templar; those that do achieve this final level of commitment are honoured in the annals of the Templar Archives. Their individualities are quashed by this act of sacrifice, so the creation of an Archon is generally a rare event.

The Archon is a rather unique unit. Because it is ethereal (neither biological or mechanical), it is not affected by spells that target only those types of units (Lockdown, Spawn Broodling, etc.). Also, it is one of the few floating units, meaning that it does not trigger Spider Mines.

Attack
The Archon attacks by generating a powerful psionic blast from its hands that inflicts splash damage. This attack is short-ranged and may be confused with a melee attack. However because of the Archon's size, it can affect flying units as well as ground units.

Gas Cost
An Archon is not warped in from a building like most units, it is warped by merging two High Templar. This, however, makes it very expensive in terms of gas. While the Archon Warp itself is free, the two templar cost 50 minerals and 150 gas, bringing its total cost to 100/300.

Shields
The other oddity is that it has 350 shields but only 10 hit points. As such, it is rarely used against Terran opponents because an EMP Shockwave, which eliminates all shields, weakens the Archon tremendously.

Upgrades

 * Ground Weapons
 * Ground Armor
 * Plasma Shields

Strategies
There are very few specific strategies involving Archons. Generally, they are used as support troops versus Protoss and Zerg opponents. They are very good at killing Zerglings and are almost absolutely necessary to counter the Ultraling combo because they splash.