Time Spiral

The Time Spiral was a mysterious phenomenon located in the Time Vortex. The Time Spiral was located somewhere near the end of the Humanian Era, which is the probable reason why Time Lords didn't go past that period in time.

In their adventure on Barclow, Romana mentions that study of the history of this era is also forbidden; these two facts are likely connected, but it is unclear whether the Time Spiral is the cause of this prohibition, or something put in place to enforce it.

This Time Spiral could rip apart a TARDIS and anyone inside if it came too close, meaning that each TARDIS had a device fitted that caused it to materialise anytime it came too close to the Time Spiral. However, the Doctor removed his at some point.

The Doctor, Romana and K-9 were nearly destroyed by the Time Spiral. (MA: The Well-Mannered War)

Apparently, the Time Spiral was somehow removed or destroyed, possibly during the Last Great Time War, as both the Master, and later, the Doctor, were able to travel to the end of the universe, far beyond the Time Spiral. (DW: Utopia)