From Halopedia, the Halo wiki
The Unggoy serve as a prominent Covenant infantry force and enemy of the player in all of the Halo games.
Grunts usually fight in groups consisting of mostly low ranked Grunts, one or two higher ranked Grunts, and an Elite or Brute leader. Most of the time, when their leader is dead, Grunts will run away, even the higher-ranking ones. Typically, Grunts fight by having the Minor Grunts combine their firepower to soften and lower the enemy's shields, so that their more dangerous allies can finish off enemy units more easily. Small groups of Grunts are generally used to slow enemy movements or deal minor damage to weaken enemy forces, and force them to waste ammunition and time. There are times they take advantage of their small size; they can easily hide and ambush or surprise their enemies with an overcharged plasma bolt or grenade. Grunts working in cooperation with Jackals are very good at seizing the initiative to flank unsuspecting enemies as well.
Grunts are near always armed with the ubiquitous Plasma Pistol, although they can be seen wielding Needlers or other one handed weapons. All Grunts are equipped with, and often use, Plasma Grenade, although higher-ranking Grunts such as Ultra Grunts and Special Operations Grunts throw grenades in combat more often and with better accuracy. With the Catch skull enabled, Grunt Heavies have been observed throwing upwards of six plasma grenades. They have been known to "spam" Plasma Grenades on the Legendary difficulty, which means that in a group of 10 Grunts, as many as eight of them will throw Plasma Grenades, one after the other, in an attempt to easily dispatch their foes.
The most dangerous grunts are Special Operations Grunts and Grunt Heavies, by virtue of bearing heavy weapons such as the Fuel Rod Gun and Pek-pattern plasma cannon or a piloting Ghost or Wraith turrets. These enemies should be priority targets, as they can deal immense damage while at the same time being relatively easy to kill. Starting from Halo: Reach and onwards, high ranking grunts also overcharge their plasma pistols, becoming potentially dangerous to vehicles.
From Halo 3 and onward, Grunts have a tendency to arm two Plasma Grenades and rush to the player immolating themselves in an explosion, as a result of their Elite or Brute leader being killed in a brutal way (ie, being stuck with a plasma grenade, shot in the head or blown up by a Needler). These Grunts are generally referred to as "Kamikaze Grunts" or "Suicide Grunts". While dangerous if not noticed, a quick player can use the suicidal grunt's explosion to finish off any enemies that may be close to him if they kill them quickly. Rarely, grunts are encountered as already suicidal; this will be noted by the game's dialogue.
In Halo Wars, the Grunt Squad serves as the main infantry unit for the Covenant army, as goes with all games. On default, the squad is composed of three Unggoy commanded by either an Elite Minor, or a Brute Minor, depending on whichever individual was selected to be the leader unit for the game. They are the direct counterpart to the "Marine Squad" unit of the UNSC, serving as an all-round infantry unit. Though they may be inferior to "Marine Squads", they do move slightly faster. As with all Covenant units, the Grunt Squad is inferior to the Marine Squad, in both armament and skill. Being the main infantry unit, the Grunt Squad is effective against most types of units due to its varied armament. Their standard, primary weapon is the plasma pistol, and their secondary weapon/special ability is the plasma grenade, which is much more effective against light vehicles and even armor. Grunt squads cost 100 resources, a tech level of 0, a population of 1, and can be built at a Hall. They function best in very large mobs, where their combined fire plus the overwhelming amount of plasma grenades can be a valuable asset. This is at its best when assaulting an enemy base.
As with all Halo Wars units, the Grunt Squad can receive three upgrades to significantly increase their combat effectiveness. The upgrades are in order as follows:
Suicide Grunts are a special forces unit in the game, starting with a plasma pistol, being the leader-specific unit of the Arbiter. These Grunts have large, cylindrical methane tanks on their backs which hold far more of the gas than other Grunt's tanks. These tanks are used as explosives. The grunts charge at their enemies and detonate their methane bombs, causing massive splash damage. An example of this is the fact that at least 10 fully-upgraded Suicide Grunts can destroy a UNSC base. Suicide Grunts will also charge their plasma pistols, making them very effective against the Prophet of Regret, the Spartans, Type-26 Wraith and Type-32 Ghosts with shields and Shield Generators.
Halo Wars 2
Grunt Squad upgrade: Shrapnel Mines
Grunt Squad upgrade: Pack Brother
Grunt Squad upgrade: Active Camo
Grunt Squad upgrade: Squad Reserves
Grunt Squads can be built by all Banished leaders at the Banished outpost. Unupgraded Grunts perform okay against infantry and aircraft but poorly against vehicles and structures. Once the Pack Brother or Squad Reserves upgrade is researched Grunt squads will fair okay against vehicles and structures. Elite-led Grunt squads perform better at ranged combat. In Blitz Grunt Squads cost 10 energy, Prowling Grunts cost 40 energy, Bloodfuel Grunts cost 50 energy, and Grunt Mobs cost 60 energy.
Changes from Halo CE to Halo 2
Changes from Halo 2 to Halo 3 and Halo 3 ODST
Changes from Halo 3 ODST to Halo Reach
Changes from Halo Reach to Halo 4
Changes from Halo 4 to Halo 5 Guardians
Changes from Halo 5 Guardians to Halo Infinite