The Dead Lands of Athas
These motes of crystallized time allow for a second chance at a single action. Each shard allows its user to reroll 1 skill or saving throw roll, before success or failure is determined.
Over 1500 years ago, during the Preserver Purge, those within the Euclidean wizard fortress sought any recourse to save them from the armies of the Warlord Irikos. Their experiments led to a colossal accident which killed or trapped many within the fortress’ confines. These temporal shards were a byproduct of this accident.