Hard invisible mobs

My character lost HP very quickly, I only managed to press teleport. But the thing is, I saw my character standing in a safe place, and my friend saw a completely different picture. After my character died, my friend said that there was no free space in the dungeon, and where I was standing there were not 3 elite monsters in front of me, but more than 20 monsters around me. Invisible mobs is a new mechanic?