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.
The Grunt can use any sort of weapon that is designed to be held in one hand. In some situations grunts have been seen to use heavy artillery weapons, such as the Fuel Rod Gun. So, on higher difficulties, the Grunt can prove to be a dangerous enemy to the Player and his/her allies.
Grunts usually fight in groups consisting of mostly Minor Grunts, one or two Major Grunts, and an Elite leading them in Halo: Combat Evolved and Halo 2 or a Brute in Halo 3. Most of the time, when the Elite or Brute leading them is dead, the Grunts will run away, even the higher-ranking ones. Grunts have a typical tactical style in combat, in which the Minor Grunts combine their firepower to soften and lower the enemies shields, so that the Major Grunts, Jackal Snipers, Elite or Brute leader(s) will easily kill or quickly finishing off enemy units. 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. Grunts working in cooperation with Jackals are very good at seizing the initiative to flank unsuspecting enemies as well.
Grunts can be tenacious fighters as long as they have someone leading them. Higher-ranking Grunts may not run away, but instead, keep fighting after the leader has fallen. They are vicious little creatures and when they are determined and motivated can be deadly; especially when lead by a high ranking elite. Occasionally, grunts will commit suicide be detonating two plasma grenades in close proximity to their target. They also have a tendency to over charge their plasma pistols, which drains the battery significantly, but can completely drain an enemies shields, or cause vehicles to temporarily stop working.
Grunts are often armed with the ubiquitous Plasma Pistol, although they are often seen wielding Needlers and Fuel Rod Guns. Their combined fire, even with weaker plasma pistols, can quickly erode an enemy's shields. Grunts are almost never seen equipped with two-handed weapons like Covenant Carbines, Brute Shots, Type-50 particle beam rifle, etc. Special Operations grunts and Grunt Heavies are occasionally seen bearing Fuel Rod Guns. Grunt Heavies can carry folded Plasma cannons. On The Storm and The Ark, a Grunt can be seen with a Brute Spiker. If a Grunt is in a Phantom that is shot down, and somehow manages to survive, it will sometimes be wielding a Spiker.
All Grunts are equipped with, and often use, Plasma Grenades, although higher-ranking Grunts such as Ultra Grunts, Special Operations Grunts and Grunt Majors throw grenades in combat more often. 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.
In Halo 3 and onward, Grunts have a tendency to arm two Plasma Grenades and rush to the player, causing 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". In Halo Wars, Suicide Grunts are a special forces unit, starting with a plasma pistol, being only 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.
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:
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.