The Frozen Maw
It can fail without consequences. If the time runs out, the event will go straight to the Svanir shaman chief.
If you don’t touch any part of the Frozen Maw:
The lake area (and the snow covered area – Rani’s armor quest area) is covered in Icebrood (dogs and elementals) from the portals and shamans spawning them.
After 15 minutes, the Shaman Chief turns into the ice guy that you fight in his second form and waits to be defeated forever.
You can do the entire event but leave one portal alone and you’ll fail the event and see what happens (you’ll only miss one medal reward if fail). With the amount of players doing the event, you will never see that happen anymore. Before the loot update I was soloing it frequently, after the update, there are at least 10 people on my server in the area when it happens.