The Guide to Glorantha contains a fair amount of information on the Tournament of the Masters of Luck and Death. Here’s a snippet:

When Belintar’s mortal body expires, the next day select individuals, chosen by a means known only to the God-King, awake between dawn and day in a thoroughly magical Holy Country with the awareness that they are part of the Masters of Luck and Death. These individuals on the Other Side compete for magical powers and mystical insight.

The rulers of the six lands of the Holy Country and Belintar’s close companions are usually qualified to participate in the Masters of Luck and Death. Others are as well – people wake up knowing they are a candidate for the Masters of Luck and Death, but never why.

The Tournament is extremely dangerous. Most losers die during the contests or are magically crippled, although a few are known to have not only survived but strengthened by their participation. 

The winner of the Tournament of Masters of Luck and Death unites with the divine essence of Belintar and his body is overtaken by this immortal element, which flows through his whole being and dwells in it. The winner becomes a living god – Belintar the God-King. The great magical energy inexorably overtakes the mortal body, and after a number of years the God-King requires a new body. Upon death, the winner’s soul is liberated with consciousness and full powers to a blessed existence. 

The participants in the Tournament never speak of their contests and it is widely believed that powerful magical guardians protect the secrets of the Masters of Luck and Death.

There have been 21 incarnations of Belintar from when he first appeared on the shores of the Holy Country until his final disappearance in 1616. Belintar has been male, female, and even of the Elder Races. Some Lunar scholars have speculated on the apparent similarity between the transference of Belintar’s divine essence to the winner of the Tournament of the Masters of Luck and Death and the various Masks of the Red Emperor; however, the Red Emperor himself strongly rejected such comparison, and had the scholars in question strangled for their blasphemy.