Lethian's Crossing runic hall is a location in Tyranny. It allows access to Sunset Spire, once the correct glowing pattern is drawn.
Sunset Spire is the only Spire that can be unlocked without much hassle, if you skipped Lethian's Crossing and attacked other regions (eg. choosing Burning Library over the Crossing after securing Blade Grave for the Disfavored). In fact, Sunset Spire could very well be the first Spire you unlock in Act II, as the entrances to the runic hall and the Oldwalls are open (and you can find 2 charcoal rubbings to get a good idea of the runic hall pattern); note that the entrances will be locked once the Crossing is chosen as your next attack location.
Points of interest
Approach the pattern on the floor. Replicate the following pattern (you draw lines by selecting two points you want to connect):
Activate the teleporter. Once activated, the teleporter allows map travel.