Mighty Fortress

8th-Level Conjuration
Casting Time: 1 minute
Range: 1 mile
Components: VSM($) (a diamond worth at least 500 gp, which the spell consumes)
Duration: Instantaneous

As in Xanathar's Guide to Everything except the following has been added:
At Higher Levels. When you cast this spell using a spell slot of 9th level, the fortress lasts for 30 days. Casting the spell at this level once a month for a year makes the fortress permanent.

Source: XGE 161

Changelog