Adonalsium
Who or what Adonalsium is unknown but there is evidence that at one time he was the God of the Cosmere. until it (Adonalsium) shattered. Hoid and possibly the Shardholders were present at this. Its essence is currently in pieces known as Shards of Adonalsium. There is evidence that there are 16 shards but no concrete proof.
Shards of Adonalsium
After Adonalsium shattered into its present form the pieces were given to or taken by individuals that then had the powers of the aspect of the Shard of Adonalsium. the shard then shaped the person that held it to further embody the nature of that piece of Adonalsium. After the Shard shaped its holder they could only do that which was part of the asspect of there shard. For example in The Way of Kingson page 275 U.S. Hardback it says "Ati was once a kind and generous man, and you saw what became of him." Ati was the name of the person who held the Ruin-Shard before being killed by Vin holding the Preservation-Shard at the end of The Hero of Ages. Intentionally killing Ati went against the nature of the Preservation-Shard but as Vin had not yet become attuned to it she was able to use its power to her own ends.
Named Shards:
Ruin,
and Austere
Shardholders:
Aona,
Skai,
Ati,
and The Almighty
Are or were, at one time, Shardsholders. The names of the Shards that Aona, Skai, and The Almighty held are unknown. in addition Sazed holds the Shards of both Ruin and Preservation but his current Shard's name unknown.