Air carrier flight crew received a caution obstacle aural warning on approach. Flight crew corrected flight path and landed uneventfully.

Narrative

Going into CRW at night; landing runway 23; the weather was clear; we briefed everything; ILS23; visual; and all other recommendations; and stated that we would not accept a visual from ATC; requested vectors unto the ILS. On base leg we noticed there was a GS warning so we asked approach; by this time we had joined the localizer; we told him we had the field and vasi and he cleared us for visual; went to tower; asked him and we were the first to notice the GS was out. We were fully configured by the final approach fix and started down. We had briefed a visual approach backed up by an ILS. So when I started down I put a higher number in VS mode. Before I could correct it we got a caution obstacle aural warning. I disconnected the autopilot and stopped descent; this corrected the flight path and the warning went away. We followed the VASI and landed normally. GS was not listed as out of service on paperwork or on ATIS. We noticed it and instead of selecting 800fpm at FAF the rate of descent was higher. During this time FO (First Officer) was checking in with tower and inquiring about GS. This happened shortly after passing the FAP (Final Approach Point) fix. We got a caution obstacle warning.We should have asked for delay vectors and briefed a LOC only approach or GPS as backup to the visual approach; but since we were on LOC and had the VASI and field we elected to follow that.