Antlion Guardian

The Antlion Guardian is a variant of the Antlion Guard encountered in Half-Life 2: Episode Two. Apparently, only one Guardian is present in each hive near the larval clusters. The Guardian patrols the "Nectarium", protecting the larvae (The extract of which is used by Vortigaunts to manipulate the Vortessence). Guardians are also very protective of Antlion Grubs, which are premature Antlions, and so fight ferociously to protect them. The Guardian becomes enraged if the larval extract is taken and the Guard may pursue the thieves for some distance. In Episode Two, a Guardian follows Gordon Freeman (whose Vortigaunt companion had taken the extract) outside and battles him.

Guardians appear to be the hive 'queens' (although it is unknown if Antlions have genders) that produce the larvae, Vortigaunts have learned to keep them alive when raiding the nest as once they are killed, they emit an odor (that humans cannot smell) that effects the larvae in some unspecified way rendering them useless for extracting.

Vortigaunts refer to Guardians as the "Ancient Guardians".

Capabilities
Guardians are basically an Antlion Guard that produces a deadly neurotoxin (somewhat like that of the Poison Headcrab), however, when you fight the Guardian outside in Freeman Pontifex, it doesn't have its neurotoxin effect.

Tactics
When fighting the Guardian, be sure to keep a good distance away from it. Its headbutt attack is quite powerful, and can send you flying quite a distance &mdash; or result in instant death if you are pinned against a wall. These attacks also have a poisonous side effect, which reduces the player's health, regardless of the current amount, to 12 &mdash; but the Guardian, unlike the poison headcrab, can actually kill the player if additional damage is caused.

When the Guardian chases you through the hive tunnels, do not attempt to fight it or you will fail the mission. The narrow tunnels don't offer much room to dodge around the guard and avoid its charge like usual so you should always take cover in the 'grub tunnels' instead, be aware that the guard is faster than you, even while sprinting. If necessary, you can try to use the SMG's secondary grenade which will stun it for a few seconds if you can hit it in the midsection (be wary of inflicting fatal damage though, and keep in mind that you may need one to quickly break through the barrier at the end).

When you encounter it again outside, you will have to kill it &mdash; be aware that damage caused underground will not persist when you meet it topside.

Trivia

 * The Orange Box Prima Guide also states that Antlion Guardians are Antlion Guards that have been mutated.
 * The Antlion Guardian is essentially just a reskinned Guard.