Nyriki's portal is the portal used to teleport to Nyriki to start playing the Big Chinchompa activity. You can find this portal in the Tree Gnome Stronghold south of the Gnome Agility Course upstairs at Mieliki Tapio's place.
There is a "Check" option on the portal to see when Nyriki is due to emerge at a Hunter area around Gielinor. This will also tell you where he will emerge. Then, when Nyriki has appeared you can teleport straight to him and start the activity. (You can teleport to the spot where Nyriki will appear before he appears too if you would like to do that.)
Note: You can only use this portal 3 times a day.