"Somebody call for an Earth-shaker!"
Basilisk is the most common long range artillery unit used in the Imperial Guard. Despite the fact that these self-propelled cannons are extremely fragile and would never withstand a direct battle with the enemy, they can still turn the tide of battle, especially when placed on the elevation far away from it and able of receiving targeting information from the frontlines. They fire high-explosive projectiles that devastate both infantry and light armor of any origin, placing fear in the hearts of enemies and effectively killing their morale.
Lone Basilisk may not be much when it comes to defending against the armor assault, due to its low firing speed and limited cone of view, but, as it often happens in the Imperial Guard, these machines begin to excel in symbiosis with other units, especially Snipers of Heavy Weapons Teams and/or Vindicare Assassins.
In game Basilisks are Tier-II units of the Imperial Guard and usually take place as the designated artillery for the second half of the battle. Due to the ability of the IG to advance quite fast, they usually are the first artillery unit in the field, even though most of the time they are nothing but enhancement of base defences because of their low HP and speed, which prevents of their use in the skirmishes. During the big assaults on enemy base they can be moved out of safe zones and used against base defences, even though it is recommended to keep a sizeable force near them to prevent a surprise counter-attack. Usually, deploying Basilisk on small maps leads to the expansion of the controlled territory for Imperial Guard and their allies.
Basilisk artillery is among those, that posess the ability to fire an Earth-shaker round - one of the most powerful artillery projectile in Imperium arsenal. Earth-shakers bypass any defensive additions like Tau shields and deal devastating damage directly to the target, which almost always leads to its destruction. They proved their worth against almost every enemy of the Imperium, especially Orcs and Tyranids, which are known to attack both in big numbers and quite tight formations (if Horde and Swarm can be counted as a "formation" at all) - something the Earth-shaker extremely approves of.
In-game, Earth-shakers are special ability for Basilisks and have both cooldown timer and resource cost. Even though the area of effect is quite small, compared to the special abilities of other artillery units in the game, Earth-shakers compensate it with their extreme power, devastating any low-tier units in area and seriously damaging almost everything else.