I actually had been intending to do a similar sort of cosmic horror spark with the Cthulhu CoinSides involving the cipher part and the glow in the dark aspect, but your Spark seems much more thought out and better integrated with the functions of the coin as a whole! I also love the selection of the headings, really adds to the tone and atmosphere of the Spark.
I feel like the ritual goal and events around Yvain's death perhaps should be laid out at the beginning of the Spark for the benefit of the DM/GM, as it can potentially get a bit confusing as to the order of events and what is meant to happen when and how precisely the summoning and offering is supposed to work...or just detail that these are all elements the DM/GM can choose to include or not. I do love the open ended aspect of it!