(Sorry for bad English) I found solution for this situation. Run BACK towards the beam over the pit, but not too fast. The idea is that you MUST die above the pit just before stepping onto the lighted part of the beam. Convulsing Senua will roll into the light. If your dying animation will end on a lit part of the beam (it looks strange, but you'll not fall into a pit during this animation) then monster will not be there during your next respawn. You'll get a new torch before next rune-sealed door.
P.S.: Before jumping down to that next door, don't miss lorestone near your spawn point.
Post edited August 12, 2017 by ssv_170379