Source: Devil at the Dreaming Palace, pg(s). 65
At some point, Kharnas stole and utilised a powerful life-giving artifact called the Radiant Spark, which he used during the siege. He was ultimately vanquished by the defenders, although his ill-gotten artifact was lost and never recovered.
In observance of this lost relic and to honour the memory of the Absalom defenders who repelled the so-called Angel-binder, a memorial was established known as the Radiant Festival which began the following year, 1620 AR.