If you cannot enter then that might be a bug, but otherwise I'd try jumping down the well.
Edit: On only one or two occasions I've seen DESCENDER hung up in mid-air...it could be a number of things, it probably needs a bump or something, but it's so mysterious why these things happen TBH. It works 99% and then something weird happens.
I've been looking and it could perhaps be that the bones are "blocking it", so I'm adjusting something there and will keep an eye out in my own playthroughs.
I would recommend leaving the screen and returning, and if the creature still hangs in the air, load an earlier save.
It is a rare bug but that's a couple of times, now.
Edit: That your DESCENDER is still pink in your screenshot is of interest. If you've killed it twice, it should appear RED; 4 times, black.
After 4 kills it will notify you about a WELL, and attack you relentlessly until you DESCEND THE WELL, which will be accessible.
Lbh pna qvfpbhentr GUR GUVAT jvgu PYBL PBZOHFGVOYR, znqr jvgu Gbjreyhat naq Vaprafr.
Rqvg: Guvf "gvc" vf nyfb sbhaq va gur VAPRAFR WBHEANY lbh svaq va gur GBJAUBZR.
Rqvg2: Lbh pna nyfb chepunfr PYBL PBZOHFGVOYRF sebz gur FNYG FRYYRE.
Rqvg3: Vs vg arire "pngpurf lbh" ng 40 QVIVAVAT, vg jba'g sbyybj lbh UBZR.
Edit5: THE DESCENDER is one of the oldest events in TOWN by a long shot and as such I dare say it is perhaps inelegant by my current standards, a full 2 years of development on.
In some ways I suspect it might be worth redoing the DESCENDER entity, as it is fairly complex and sometimes behaves in unexpected ways, but I've thus far tried to maintain it and force it to behave more intelligently as times has gone on.
Why some events only sometimes behave certain ways I cannot wrap my mind around.
