its called roulette

[Eye_Gore]

its 50% if all 4 people are rouletting but it also searches for people queued up and most just queue for neverreep influencing the roulette system, not only this but i think in 3.1 they will keep it in the expert unless there is a new tome stone added in then it will probably be daily roulette heavenward

Its 50% all the time, either you get it or you don't.