To summon 艾克索斯，统御先驱
you need combinate 3 items to create 统御之召
I got all items on my first try.
With 3 other players, stand on the corners of the platform and cast 痛楚集会
to summon these rares.
/way 28,9, 11,6 Talaporas, Herald of Pain
/way 19,4, 41,4 Apholeias, Herald of Loss
/way 42,1, 21,2 Ekphoras, Herald of Grief艾克索斯，统御先驱
is located on Altar of Domination (upper level) - The Maw.
To enter on Altar of Domination you need open a Gate in Perdition Hold.
To open the Gate you need the achievement 麻烦上身！
(click on this achieve for more info).
/way 33,0, 57,0 Gate 1
/way 34,9, 66,4 Gate 2
After open the gate, run to the rare location on Altar of Domination and use 统御之召
on the center of the room.
/way 23,0, 68,4 (cave entrance)
/way 46,0, 47,7 center of the room
After use 统御之召
will appear a portal on the center of the room.
Enter on this portal to reach Altar of Domination (upper level) and kill the rare.
/way 20,6, 69,5 Exos, Herald of Domination