Hazelmere's Signet ring is a rare drop from the Super Rare section of the RDT and it requires you to have Tier 4 luck active in order to get it to drop. It is, essentially, a much (much) rarer Vecna Skull.
Whilst the drop rate is far too rare, you would be surprised at the amount of people who don't wear LotD whilst killing slayer creatures. I've been looking at data regarding failed rolls for the ring in the past couple of weeks and it surprised me.
source:
tldr: slay with ring of dwarves on or camp something with good rdt chance (abby demons, dark beasts) and try get a 1/100000000000000000000000000 drop chance then sell ring for 10b and buy a nice partyhat