A practitioner who has earned the confidence of local adepts in a major metropolis may given a sealed envelope with an address in a deserted district—and an indicated time deep in the black hours.
The sealed envelope will contain a key to the door at that address.
The door will lead to a stair and that stair will lead down and down beneath the city.
Eventually there will be another door, not locked, and inside a chamber.
(The difference between a chamber and a mere room is that chambers are private.)
The chamber is full of mages. Users of arcane magic of every stripe. They are gambling.
They occupy the circumference of a stone circle, in the center of which is a bloodstained summoning sigil.
Into this ring comes a competitor, she whispers to the master of ceremonies her name and the powers to which she aspires.
The master announces then ”Here is Vasylaa of Thrazz, who seeks secrets of the Eigth Octacle!”
The crowd roars and exchanges bets, the sigil begins, blackly, to bubble.
From the center comes some creature of the outer spheres, whose power is commensurate with the power Vasylaa seeks.
They duel, of course, to the death. If she wins, she gains sorcerous knowledge from the dimension from which the creature came—if she loses, her cares are at an end.
In Game Terms
State the level of the new spell you wish to gain—it must be at a level you are able to cast.
Then step into the ring and an extradimensional creature will be summoned randomly. It will have hit dice equal to the lowest PC level at which a standard magic-user is able to cast the spell level desired. It will have abilities to match.
The Lamentations of the Flame Princess Summon spell works well enough for this, though other options are fine so long as it’s random and can spit out a sufficiently interesting and powerful creature. If using LotFP, the initial magic save is also an initiative roll—if the magic-user makes the save they get initiative in the first round.
Good luck wizards.