klakkat said:
In say, the Warhammer (and 40K) settings, the gods are very rare and are much more truly immortal and would be an unthinkable challenge; even the other gods likely couldn't kill one (as evidenced by the 4 Chaos gods so far being unable to slaughter Sigmar, which I'm sure they'd love to do).
[specialist subject alert]
But:
-The C'tan ate each other (Night Bringer and the Outsider chowed down the most)
-Slaanesh killed and ate the majority of the Eldar gods (Vaal, Isha, etc)
-Slaanesh shattered Kaela Mensha Khaine into shards
-The Deceiver and the Laughing God regularly war with each other
-The Chaos gods are immaterial and can only interact with the material world in limited ways, Sigmar was material before ascending to Godhood
-The Old Ones warred with the C'tan
-The Emperor is actually not one being but a hive-mind entity consisting of the millions of psykers sacrificed to it guided by the Emperor's titanic intellect, he is not even whole anymore having cast out a part of himself in order to kill Horus
You are right though it is an unthinkable challenge for a mortal to kill a God because the majority of Gods exist in the immaterium: a realm that exists in the same space as the material but is not really physically connected to it (only by a few entrances). The realm itself is impossible to traverse for the ordinary mortal unaided as it has no fixed form.
In that continuity Gods are formed by the beliefs and actions of people, the more people focus and believe something to be powerful the more powerful it is. Souls can also join to make a God (like the embryonic Eldar God of spirit stones).