How to get to Crypt of Sul
In order to get to Crypt of Sul you'll first need to travel through Plane of Health to reach Sul Vius: Demiplane of Life. Once in this zone you'll want to travel north into the Mayor's Building where you'll find a large table with a skull sitting atop of it in the center room.
Click on the skull to zone into Crypt of Sul. There's also a skull sitting ontop of the same table in Sul Vius: Demiplane of Decay - clicking on that skull will grant you access to Crypt of Sul as well.