This is, quite simply, a virtuoso treatment of bitterness in all its glory. The astringent bitter of wormwood; the damp, earthy bitter of geranium; the sweet, raunchy bitter of patchouli.
And all underneath, there is castoreum. Oh yes yes god yes castoreum. Some prefer to their animalic kicks from whale snot (ambergris) or cat pee (civet) or the ol' reliable deer loins (musk). But there really is nothing quite like the raunch of the mature beaver's castor sac.
Leather is a bit player here. To my nose at least it's basically indistinguishable on its own. What it does do is blend into the castoreum and lend it some respectability. That and the symphony of bitter herbals let the magic beaver juice do its thing unobtrusively.