Mass Effect Legendary Edition

Some time after completing Horizon, Kelly will let you know that the Illusive Man wants to speak to you in the conference room. When you go there, he tells you that and he wants you to investigate.

Be aware that as soon as this mission triggers, you won't be able to access the Galaxy map and so you'll have no choice but to talk to the Illusive Man and complete the mission before you can do any other missions (it won't prevent you from completing any of these missions later though). It is not exactly clear what conditions must be met to make this mission trigger, but it seems that in most cases:

▪after completing Horizon, a 5-mission countdown begins and;
▪you need to have recruited at least 8 squad members.

Plan accordingly as this will be one of the harder missions in the game, so you might want to make the most of the xp you can gain from those 5 missions to be as strong as possible.

Especially when playing on the Insanity difficulty, I suggest you take Miranda and Mordin with you as you'll face a lot of enemies with barriers and armor. Among them are Miranda's warp combined with the Collector Particle Beam will make your life a bit easier and make relatively quick work of their barriers. As soon as you complete the mission and finish debriefing with the Illusive Man, the achievement will be yours!