War Assets/Krogan

The krogan clans have agreed to throw their considerable might against the Reapers. Their shock troops are aggressive, relentless, and recklessly brave.

Urdnot Wrex
Requires: Not killing him on Virmire in the first Mass Effect game.

Urdnot Wrex fought alongside Commander Shepard during the Reapers' first invasion attempt in 2183. Afterwards, he returned to Tuchanka and spent the following years rising to the leadership of his clan, championing progressive methods meant to strengthen the krogan as a whole. Conservative elements were unhappy, but news of a successfully cured genophage has solidly united all of Tuchanka under Wrex's command. Hailed not only as a great warrior but a savior, the krogan will follow Urdnot Wrex into the maws of the Reapers themselves.

Should you kill Wrex on Virmire, you will instead get Urdnot Wreav as a War Asset, who is worth 5 TMS less. ( Since this information comes from coalesced.bin it still requires confirmation. )

Grunt
Requires: Survival of Mass Effect 2's Suicide Mission, and the completion of the quest Attican Traverse: Krogan Team

"The perfect krogan" was thrown into the world without a past, name, or a cause. After destroying the Collectors with Commander Shepard, Grunt returned to Tuchanka to learn what it meant to be krogan. After countless arguments and brawls, the clans have granted Grunt a modicum of acceptance. Tough, wary, and cunning, the young krogan knows this experience will pay off now that it's time to fight the Reapers' forces.

Aralakh Company
Requires: The completion of the quest Attican Traverse: Krogan Team

Ancient Tuchankan legends claim that the Aralakh Wastes, named after the system's star, are where the first warrior died in battle. Aralakh Company is made up of krogan from all over Tuchanka, each chosen for their strength, ferocity, and fighting experience. No krogan community would dream of sending anything less than its finest warriors to represent their clan. The honor of who's to be chosen for the company has started more than one brawl.

UPDATED (If Grunt survived the Suicide Mission)

(according to game files only)

Aralakh Company is led by Grunt, a genetically engineering "perfect soldier" who fought the Collectors with Commander Shepard. While young, the krogan has survived such brutal battles that the older warriors of the group have begrudgingly accepted his command.

UPDATED (If Grunt: Rite of Passage was completed)

Grunt was appointed representative of Clan Urdnot in addition to becoming head of Aralakh Company, a political appointment not lost on the other clans. Grunt sees this new positions as the first large victory he has won on his own--the respect of his people is difficult to come by.

UPDATED (If the rachni breeder or queen is saved during Attican Traverse: Krogan Team)

Wave after wave of rachni overwhelmed the Aralakh Company on Utukku. While fighting an impressive enemy to the death is considered an honorable end for a krogan, Aralakh Company has effectively been wiped out.

Krogan First Division
The krogan First Division is a conglomeration of infantry and vehicles hastily organized into a single coalition force. The chain of command is divided by clan, with representatives from each serving as officers. Despite ancient differences, fighting the Reapers together is building a spirit of camaraderie among the division's warriors. Krogan from different clans are starting to swap equipment, stories and even the occasional barel of ryncol.

Krogan Clans
Centuries of krogan infighting abruptly halted with the news of a cure for the genophage. With their future restored, Tuchkanka's clans have united in a show of force not seen since the Rachni War. The clans have also been stockpiling weapons of mass destruction for decades. Thankfully, their full destructive force can now be turned on the Reapers instead of Tuchanka. Their strength, size, and regenerative powers make the krogan ideal shock troops.

UPDATED (If Paragon option during interview with Allers after Priority: Tuchanka)

Commander Shepard's interview with Diana Allers about bringing the krogan into the Reaper War cast the krogan in a favorable light, which drew even more of the clans to the allied forces' banner.

UPDATED (if the player completed Citadel: Kakliosaur Fossil)

Fierce, toxin-immune creatures known as kakliosaurs, once used as battle-mounts by the krogan, have been cloned from the fossilized kakliosaur skull discovered by Commander Shepard.

UPDATED (if Maelon's data in Mass Effect 2 was destroyed causing Eve to later die of complications resulting from a cure based off incomplete data)

With the death of Eve, the Clan Urdnot shaman who provided the basis for a genophage cure, several of the krogan clans have fallen into conflict over who was responsible for her death. The fighting, an excuse for many to act on long-standing feuds, has significantly weakened krogan forces.

Clan Urdnot
Clan Urdnot has become the unofficial, but uncontested, representative body of krogan power to the rest of the galaxy. Within the past decade, Urdnot's warriors have either absorbed or wiped out Clan Nakmor, Jurdon, Quash and even the legendarily ferocious Clan Raik. The discovery of a fertile Urdnot female rallied even more krogan, regardless of affiliation, to Urdnot's banner.

Krogan Mercenaries
Many krogan went fortune-seeking out in the lawless Terminus systems. Their imposing bulk and infamous brutality won them prized positions as bodyguards and mercenaries. When news of a genophage cure spread, these drifters were contacted by their clans and urged to join Shepard's forces against the Reapers. After centuries of aimless brawls and bleak futures, krogan everywhere are heeding the call.