AKA The throw-out-your-pure-essence-for-crap minigame
Basically, you run through a cave full of "high leveled" monsters or go through a long path and squeeze through a mile-wide hole, then you go to some broken-down altar and push your essences through the slot, then you receive a crappy rune for each of the pure essences, which (if you can possibly get 20 of one kind,) you can use the banker that was not there because he wanted to be.
To me, this seems like a scam. Jagex takes your pure essence and gives you double the exp but a tenth of the runes and no control over which ones you get. The only good part I can see here is that you have a chance of getting a soul/blood rune. I see no point.