My take on a Skeleton Commander
Art is taken from Xu-Ifnit, Osteoharmonist full art by Ashley Mackenzie.
Mechanics:
I wanted to do a skeleton tribal commander that wasn't a generic "skeletons you control get X if Y". Design wise, Gravestorm (GS) triggers well with the modern skeleton archetype of self recurring allowing the player to sacrifice a card like [[Tenacious Dead]] to [[Ashnod's Altar]] or [[Phyrexian Altar]] to recoup some mana and reanimate loop the creature to up the GS count.
Once you have achieved the desired count, you cast Gerisax and get 3-4 copies. Which allows you to mill yourself to get more loop enablers in your graveyard, or to put your own bombs in to reanimate later. It also allows you to mill and exile your opponents bombs.
After those cards are in exile you sacrifice the extra copies of Gerisax and trigger his final ability which lets you return all but one of the cards you exiled to the battlefield. This gives you wiggle room to keep threats exiled since the reanimation trigger is non-optional.
I picked a 4/2 stat block so that he presented a clock if he remained on board, he would still be easy to remove unless you are playing Skeleton lord cards to augment his statline, and the statlines of his reanimated targets
I would expect him to be played with a reanimator package since he both reanimates and mills but can prevent your opponent from reanimating the cards that he milled. Also, since his ability is an ETB, returning him to hand to cast him again with cards like [[Crawl from the Cellar]] would effectively double your GS counters which is a nice bonus for budget reanimation.
Flavor:
I imagined him as a guardian of souls who guides them beyond the veil, hence he exiles the creatures with soul counters. Once they are gone, he reanimates their bones to work as imitations of their former owners. This is shown by them returning to the battlefield without their "soul" (counter) and the low stat block since they are only puppets.
Unlike other gods of passage/death, he doesn't oppose Necromancers because of their disrespect for the dead, he just doesn't want competition.
First custom card, would love some feedback!