remove chainguard (?) lockring on fixed cup bracket
The tool is called lock ring wrench, or in non-bike contexts hook wrench or hook spanner. There are a few different sizes, measure the lock ring before buying one.

EDIT: The inner part is locked by the lock ring. Once the ring is loose, the inner part should be finger tight. If it's stuck, penetrating oil and pipe wrench or the pin spanner from other answer should help.