Nearby enemies that die have a 0.5% chance to drop a Stat Anvil at the location of their death, reduced to a 0.25% chance for non-champions. You can pick up dropped anvils by moving over them. The obtained Stat Anvils will be consumed once the shop is enabled again (after your death).
This augment page for High Roller reflects public Arena Mayhem data for patch 26.19.
Graded against the other augments of its rarity, from all champions' games. The source publishes no per-champion results for augments.
See the Prismatic tier list