Fudge, Am I the only one wondering' what happened in real life?, I mean Momonga haven't eaten, since first chapter actuallyΣ(  ̄□ ̄||)...
Its because his physical form is now undead :) i think he narrate it on anime
Non spoiler: Yes, there's stuff in the LN that explains it in more detail I believe. But let's put some fact together that were in the manga as well.
1: The servers of the game shut down. Momonga was just still online when it happened. So there is no game he could have transferred to.
2: Everything around the tomb is different from the game. Only the spells this world uses are the same, and some items from the game remain as powerful relics. (like the robe that mind-controlled Shalltear or the summoning crystal that summoned the angel dominion authority)
3: Weeks already passed in the manga by now, so if there was still a body in our world he would have died already most likely.
As for a more detailed explanation: HUGE SPOILER ABOUT PLAYERS IN GENERAL IN THE LN! I haven't read it myself but others have explained what's going on:
Other players have been sent to this world before, but to different points in time and usually in groups. And they were all always so powerful that they'd conquer large parts of the world or some such. These previous players also taught this world's inhabitants the game's magic, which is why magic is the same as the game. And there's apparently a 'race' called god-kin, which are the descendants of previous players. However the previous players all eventually died because they fought among themselves and other such reasons. Players being summoned only happened rarely though, as the last players (the heroes that are talked about by the adventurer group that ainz joins for a bit) had been there around 200 years ago. Ainz is the exception since he was sent as the only player with hundreds of loyal servants that used to be npc's.
In my opinion, the one's who creepy here is the Butler .. He planned the killing of the first prince and he loves the uke. And about the black guy , I think he was like a tyrant that can't accept his feelings and shows his obsession of the uke by violence.. I don't know if I'm right