26/05/2018 10:08Posted by OnĂ¯sanI guess you need to finish your class campaign
That is correct!
The Class Hall campaign is divided in two separate parts: a first one which ends with "A Hero's Weapon" (and unlocks the third Artifact relic slot) and a second, considerably shorter one which allows the player to recruit the additional follower needed to complete "Champions of Legionfall".
Wowhead has an exceptional guide that explains the first part in great detail: you can find it here (keep in mind that the actual quest chain is hidden behind a spoiler tag that must be clicked on).
More information about the second part of the Rogue Class Hall campaign can be found here.
Have fun!