"he who returns at the doom of man" mark of the war tides. whats interesting is a war tide is when one side of a war has been wining and the other side dose something drastic to turn the tables, as we know the light has been wining, my speculation is that the darkness will unleash someone so powerful it ends our timeline as we know it, therefor we would team up with the stranger to go into the past, fighting awesome wars like the battle of six fronts, i theorize not only that if you picked a warlock that our person is osris from another timeline but that all of us are dred your, explaining why that person is a unkown class. i believe there is only one Being powerful enough to change the outcome to wins for darkness death, or in other words the grim reaper, the taker of souls. So what faction would this being belong to? well it cant be fallen, since they are so heavily damaged by us and the past actions of the vangard, it cant be the cabal they would go with a military approach, it could be the hive but something about that seems odd the hive would not be so bold to attack the tower, that leaves the vex and since the vex are based on greek mythos it makes sense.