I tried this with a legendary Chaos gate map, 1 run while not touching anything and going to the gate(did not clear map); and 1 run while not touching anything and killing everything and then going to the gate. Variables to consider is if you yourself have to touch the portal/not just teammate touching the portal.

Also this might not trigger off of a chaos gate map as well.

Update: reddit thread states that you nothing must die in the dungeon/ie you run straight to portal. I'm guessing if you run this with a group, nobody can touch anything for this achievement.

achievement best done alone

As stated in other comments, you must enter and exit a legendary secret map without killing anything. I acquired the achievement by buying Tier 1 legendary map crates with rift shards. I did not group up with others.

Geheimdungeon = Legendäre Schriftrolle aus Chaosportal.
Auch den Boss darf man nicht töten.
Schleichrobe mitnehmen, falls man angegriffen wird und Portal für Ausgang nicht nutzen kann.
Auch Teammitgieled dürfen keinen töten.