A krogan bounty hunter named Urdnot Wrex might be able to help you against Saren.
Acquisition[]
After the Citadel Council hearing, David Anderson must be asked if there are other leads that can help the treason case against Saren. He refers you to Barla Von, a Shadow Broker agent who may be able to help. In this scenario, the mission is initially named Citadel: Shadow Broker. If you didn't ask Anderson immediately after the hearing, you can do so again at his place in the Embassies.
If you are accomplishing Citadel: Garrus without doing the above, at the conclusion of that mission Garrus Vakarian attempts to persuade you to enlist Urdnot Wrex into the team. The mission name is then acquired as is, and there's no need to speak with Barla Von anymore—simply go to C-Sec Academy to find Wrex there.
Walkthrough[]
If the mission was obtained from Anderson, head to the financial district of the Presidium. Barla Von is located in the bank, next to the Emporium.
Once inside, Barla Von will tell you that he works for the Shadow Broker and the information he is about to give you would normally cost a small fortune, but he is giving it away for free. Apparently Saren has betrayed the Shadow Broker, not a good move, and now the Shadow Broker is very angry with him. You can respond to it anyway you want. Saren also conned a former Shadow Broker agent named Fist to change his allegiances to him. The Shadow Broker retaliated with hiring the krogan bounty hunter to deal with the problem.
After speaking with Barla Von, the mission name now changes to Citadel: Wrex. When you have all the information you need, head over to the elevator to C-Sec Academy. The elevator is located near the Embassies so the Fast Transit System is recommended, unless you feel like walking.
Once you are in the academy, you will see several C-Sec academy officers near a krogan off to the right of the center elevator. You may want to make a save point before going up to them, as this will lead to one of the few opportunities to switch squadmates at this stage of the game.
When you approach the group, a cutscene plays. Apparently the officers hauled the krogan in because he was accused of making threats against Fist, and they warn him off, to little effect as the krogan only responds point-blank that he will kill Fist. After a brief but tense standoff, the officers leave and the krogan turns his attention to you. After some introductions, you discover that your goals and that of the krogan, Urdnot Wrex, align enough that you agree to let him join your team.
If Garrus hasn't been recruited yet when you speak to Wrex, or you have recruited him but you haven't yet gone to Chora's Den to deal with Fist, you will not have the option of refusing to allow Wrex to join you. If you haven't recruited Garrus, Wrex will mention that there is a turian who is also going after Fist. You can then either choose to go to the Med Clinic for Citadel: Garrus or just go to Chora's Den to deal with Fist.
If you have completed Citadel: Garrus and you have already gone to Chora's Den and dealt with Fist, you can still find Wrex down at C-Sec, but he won't be having a confrontation with officers. Instead, you will find him leaning against the elevator that leads up to Dock 422 where the Normandy is moored, and if you approach (unavoidable if you want to take the elevator up), a slightly different cutscene plays. Wrex is frustrated that you handled Fist, a job which the Shadow Broker had paid Wrex to handle. Out of professional pride, Wrex states he will gift Fist's bounty to you instead. He then asks to join your team and your mission to take on Saren. When pressed on why he would want to join you, he says that he wants to be "where the action is", and that Saren portends a major conflict to come. He also expresses disgust at what he sees as the servility and greed of the other krogan who fight for Saren, claiming that his people used to have more pride.
However, you may still refuse to allow a krogan on your team, in which case a miffed Wrex says that he will hang out at Chora's Den if you change your mind; he will remain there for a large majority of the rest of the game, and you can recruit him to your mission by speaking with him there at any time. Even if you refuse Wrex's initial offer to join you, you will still receive the bounty payment, a relatively large credit reward.
Regardless of the circumstances of when you first talk to him, if Wrex joins your team, you will be able to select your squad again, and then move on.
Mass Effect 2 Consequences[]
If Wrex joined Shepard's crew, he will appear as the leader of Clan Urdnot on Tuchanka, provided that he survived Virmire: Wrex and the Genophage on Virmire.
If this mission is not completed and Wrex is never recruited, it is erroneously stated on starting a new imported game that he survived the mission on Virmire, despite not having been there. You can even ask the Illusive Man about him, and he claims Wrex returned to Tuchanka to unite the clans, which is what also happens if Wrex was recruited and survived Virmire.
However, when you travel to Tuchanka you will find Wrex's brother Wreav to be Urdnot clan leader, and he mentions his brother's apparent offscreen death. Wreav also mentions Wrex's death if it occurred on Virmire, expressing his gratitude to Shepard for removing what would have been an obstacle to Wreav assuming power.
Mass Effect 3 Consequences[]
Krogan leadership in Mass Effect 3 solely rests on whether Wrex was recruited in the first game and survived its events. If yes, Wrex continues to be the leader of his clan and the de facto leader of his species, negotiating with the other races for a cure for the genophage. He will personally assist Shepard in their missions on Sur'Kesh and Tuchanka, and he will also take up temporary residence on the Normandy SR-2. He may also reappear as a potential squad member in the missions associated with Mass Effect 3: Citadel. Finally, if he survives until the final battle against the Reapers, he will be present on Earth, personally leading krogan ground forces there.
If Wrex never joined your crew in the first game, Wreav continues to lead the krogan instead, filling in for Wrex's roles in the base game (he's not close enough with Shepard to be involved in the Citadel DLC). This has serious implications for any potential cure of the genophage because of the differences in temperament and ideology between the two brothers. It also results in concrete differences in the potential War Assets that can be gained (or lost) related to the krogan.