In my friend's vanilla server, whenever my fellow players and I enter the nether, we sometimes encounter an invisible mob (without any particles at all). My friend, who is currently in OP status, went into Spectator mode to spot it, but there's no sign of the mob at all.
I even tried to "bait" myself to see what kind of mob it was that was killing the players, and it would read either as Spider or Zombie.
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kumasasa
migrated
Having the same issue, "slain by " spider, creeper, skeleton, zombie - all around the nether portal in the nether side. The enemies aren't there and you can't attack them but you get damaged until you die and then you see the cause.
migrated
Invisible skeletons in the nether, it is a clean survival place but I found 2 skeletons inside nether fort almost killing me
There are no natural occuring spiders or zombies in the nether -> Someone spawned an invisible mob.