BIS relic combination for Assassin tanking right now is...
Dread Guard Relic of Ephemeral Mending
1) Campaign(!) Relic of the Shrouded Crusader (for situations where there are tanking "phases")
2) Elite War Hero Relic of Imperiling Serenity (for situations with steady damage profiles)
It will likely take one additional gear change to keep mitigation pool optimized when changing between a Shield/Absorb on-use relic and a passive Defense relic. I swap my body slot between ~80 Absorb and ~80 Defense accordingly.
DG Shield/Absorb relic has a 20s duration instead of 30, making it inferior to the Campaign version.
DG Absorb proc relic and EWH Defense relic are pretty much equal. If you don't want to pvp it is certainly a viable alternative.