Mutalist Alad V is the assassination target on Erris. He drops blueprints for the Mesa warframe.

In order to gain access to Mutalist Alad V you must first complete the Patient Zero quest and then craft a Mutalist Alad V Assassination Key with the blueprint you get from that quest. This blueprint requires Mutalist Alad V Nav Coordinates, which is a battle pay reward from Infested Invasions but can also be acquired from Orokin Derelict Survival and Defense missions.

Notice that Alad V is invulnerable when he has his collar equipped around his neck. During the fight he will throw the collar on a random player and mindcontrol him/her; this can wipe the entire team if a Warframe with a powerful DPS ability gets mind controlled!

I recommend using Loki to farm Mesa's blueprint as he is very fast and efficient for this fight. Use a stealth build and equip a powerful sniper rifle. Sprint/bulletjump/cloak your way through all the trash NPC's and hurry to the boss. When you get to Mutalist Alad V, cloak and place a Decoy or a Specter to keep him occupied - then aim with you sniper rifle and wait for the moment when he becomes vulnerable (i.e. when he removes his collar and either holds it in his hand or throw it at a Specter or team member), then quickly nuke him.