Endlessmode is a post-True Guide mode which occus after beating True Guide.
Well, basically, here's how it goes:
Mechanical Bosses -> Plantera or Withera -> Golem -> Lunatic Cultist -> Moon Lord
Duke Fishron -> Whale of the Mosaic -> Mushroob -> Key Master -> ??? -> ?!?!?! -> Solar Cultist -> Sun Lord
Once you beat those, there is a message which says "You did well."
Talk to the Guide. Once he says "Now you shall have to FIGHT ME!", getting another hint or leaving turns him into the True Guide, which is actually a Trapped Demon named Killa. The message announces "You have freed me! Now all of Terraria shall die!", and you have to FIGHT him, with SUPER HORRIFIED debuff, and if you TRY TO ESCAPE OR DIE, you BECOME A GHOST AND YOUR CHARACTER IS DELETED LIKE HARDCORE.
Once you beat him, he announces that "he has future plans to beat you" and fades out of existance. The World announces "The souls of the Yin-Yang is freed..".
New Essence Enemies spawn, aswell as more harder enemies.
Events spawn 20% higher. Costs are reduced, and enemies get there expertmode attacks.
New damage (aka "Abyss" and "Nature") is generated.
Once you beat him, the Guide will no longer settle down.
New Essence Bosses are spawned. They aren't optional to see the end credits.