The tentacles would be amputated. Understandably. Also the aberration is bound with Runes. It can't escape. Furthermore chain it. TAME it, Charm it. Use Chaos magic to control it, potions... contemplate and apply man, you are one of best here. Pretty certain you would find a few hundred ways... Make a deal with food as offering, it gets food when it writes a book. Now my dear Johan just imagine how much data would the aberration tell to Futen, when he would ask the questions of which we would never think of!? What magics could the aberration teach us?! spells that it could scribe, skills, treasure locations, other notable places of power.. Likely the info would not be super valuable per say, if Aberration was always locked in the cave.. But its victims of their knowledge is juicy.. and in general its not a good practice to waste resources.