Shardholder

A Shardholder is an entity who has "agreed" to embrace a Shard (potentially in effort to "control" it) or otherwise "dispense" its effects. Their personalities have all been changed from those which they originally were due to which Shard each of them took up. However, the original sixteen Shardholders all knew one another.

When an entity takes up a Shard, they take up its power and its Intent. They keep their mind, but the Shard's Intent bends their mind over long periods of time. For example, Ati - who held Ruin - was originally "a kind and generous man", but over thousands of years the Intent of his Shard twisted his mind, driving him to destroy.

This is why Preservation/Leras needed Vin to destroy Ruin. Leras had held Preservation for so long that he was incapable of any destruction; even that of his enemy. Instead, he trapped Ruin, and gave his power up so that Vin (who only held Preservation for moments) could do the deed for him. She didn't hold Preservation for very long, and was still capable of destruction.^{[citation needed]}

This is another reason that Odium is so frightening. Not only is Odium "the most frightening and terrible of all the Shards" (according to Hoid), but the man who took up the Shard (i.e., Rayse) was already a terrible person. That's a very dangerous combination.

The person (whomever s/he is) that holds Cultivation IS, in fact, Cultivation. Assuming that person has been alive with that power for thousands of years, s/he would be changed by the Intent of Cultivation; to nurture and grow things needed for the future.

Characteristics

According to Brandon, going other places (i.e., Worldhopping?) means multiple things to someone actually holding a Shard. They can exist in the Spiritual Realm, where all things are one. And they can even kind of comprehend it ... .

When a Shard gets Splintered, its Holder does not automatically get killed because one can give up pieces of Investiture.

Abilities

Also according to Brandon, each Shard has the ability to Invest in a place; each just needs to "start making some stuff."

Known Shardholders

Currently, there are twelve Holders - alive or dead - who are known, for whom their Shards are known, and for whom their worlds are known. There are five Shards for which each their Holders are not yet known.

Holder	Shard	World	Book
Almighty/Tanavast (deceased)	Honor	Roshar	The Stormlight Archive
Rayse (deceased), Taravangian	Odium	Roshar	The Stormlight Archive
Unknown	Cultivation	Roshar	The Stormlight Archive
Ati (deceased), Sazed	Ruin	Scadrial	Mistborn series
Leras (deceased), Sazed	Preservation	Scadrial	Mistborn series
Edgli	Endowment	Nalthis	Warbreaker
Aona (deceased)	Devotion	Sel	Elantris
Skai (deceased)	Dominion	Sel	Elantris
Bavadin	Autonomy	Taldain	White Sand
Uli da	Ambition	Threnody	Shadows for Silence in the Forests of Hell
Unknown	Invention	Unknown	Unknown
Unknown	Whimsy	Unknown	Unknown
Unknown	Mercy	Unknown	Unknown
Unknown	Valor	Unknown	Unknown

Sazed now holds the Shard Harmony, which is Ruin and Preservation joined.

Notes

A Shardholder is also known as the Vessel of a Shard.

Speculation

Fabrials are Cultivation's power. Technology fits that Intent perfectly.

Q&A with Brandon

Q. I am wondering if it is possible for a person to take the power of a Shard and then later decide the whole god thing isn't working out. Can they retire and go back to being a person? Or are they immediately sent to the Beyond?

A. It is possible that they could retire as a person. Wouldn't be the first thing that would happen.