Perhaps Nomad is our own successor ... or Nomad's final death is what Aeternam saw.
Sliske extracted a portion of the player's soul. (Then even merged with the player char in a way).
Sliske then took the portion of "World guardian" power he took from us and stuck it in Wight Nomad.
So right now, there are 2 World Guardians running around, technically with the same soul.
With convoluted series of events that has occurred since Guthix gave us the World Guardian power, it would be possible for Aeternam's prediction of the demise of the World Guardian to be somewhat vague.
Heck, who's to say that Nomad even gives us a way out to escape death:
1) Nomad allies with the player for some final threat
2) Nomad and the player both know that the situation will end up perma killing 1 of them. they also know that they need the full World Guardian power.
3) So Nomad offers to take on the burden himself (or just takes the power), relieving the player of the burden of the World Guardian mantle
4) Nomad goes off and dies some glorious, yet painful, death against the monstrosity threatening the universe
11-Jan-2017 19:32:15