I just killed Lagon for the first time and i was wondering what i should do next?
the game told me this is the main quest end ( at this point) yet i see people fight Rahyeh and other bosses? is this just from monoliths or am I missing something?
If you followed the quests, the Forgotten Knight brought you back to the End if Time. There, you must talk to Agony, who will direct you to the monolith. And you also will have to enter the arena.
The monolith will now be your main activity. It will bring you gold, loot, XP and blessings. Blessings are bonuses/buffs for your hero.