So I have installed the metapost macros in /usr/lib/texmf/metapost/therion/* and the TeX macros in /usr/lib/texmf/therion/* and run texhash, which has indeed updated /usr/lib/texmf/ls-R
However this seems to work fine for the metapost stuff but not the TeX stuff.
Web2C based TeX distributions (you have one such according to pdfTeX banner) use configuratin file texmf/web2c/texmf.cnf (or vartexmf/web2c/texmf.cnf -- various TeX distributions may differ in this point). All searching paths are set here. For tex is it usually TEXINPUTS.pdftex = .;$TEXMF/{pdftex,tex}/{plain,generic,}// TEXINPUTS = .;$TEXMF/tex/{generic,}// It means that pdftex searches only in 'pdftex' and 'tex' subdirectories of the texmf tree. So one solution would be to move therion tex to texmf/tex/. (You had good luck to copy metapost files to texmf/metapost/) It's also possible to modify TEXINPUTS to point to directory outside of texmf, so there would be no necessity to copy therion files in texmf directory. Try TEXINPUTS = .;$TEXMF/tex/{generic,}//;/cave/therion/lib/tex// or similar. Run texhash after changing texmf.cnf file. Best regards Martin