The Chaos portal or better known as the Chaos vortex is the entrance/exit to the realm of Chaos.
Generally it is referred to as the Chaos portal on the Pinnacle side of it, and as the Chaos vortex on the Chaos side of it.
It is located to the north of Pinnacle from CoT go: n,w,2n,e,n,enter.
A list of areas in Chaos.