This issue is a variant of The Avengers (Marvel, 1963 series) #186 [Direct]. There exist further variants.
This issue features the first appearance of a Chthon-possessed Scarlet Witch, a "Dark Scarlet Witch", so to speak. It anticipates, in many ways, the Dark Phoenix character, by the same artist, that first appeared in The X-Men #134 (June 1980).
Magneto revealed as father of Quicksilver and the Scarlet Witch via the appearance of Magda in X-Men #125. The origin of Quicksilver and the Scarlet Witch storyline discussed in detail by Mark Gruenwald in the letters page of Avengers #192. This information added 2005 by Steven Tice.