Which Portal Is This?

Portals have a 50% chance of spawning on a normal world and 1 to 2 spawn on Nightmare. You can find it randomly or by using a Scroll of Portal Location which reveals the portal's location on your map. If you use the scroll it will show up on the map as a purple circle and will highlight the area as if you've been there before. When you reach the portal all you need to do is interact with it. Inside you will find plenty of useful items.