Congrats on making your first EDH deck! EDH is a great format, especially if other new people are playing it with you.

I saw Sedris and immediately thought "Graveyard abuse!", but you also have a good number of cards that interact well with instants and sorcery. EDH decks are big enough to support different themes, but remember that Melek and Hypersonic Dragon get better the more instants/sorceries in your deck.

Some good cross synergy with your general and all the big creatures in this deck is Reanimation and self mill/discard. Luckily black has many ways to do the first (Torrent of Souls , Footsteps of the Goryo ), and blue has plenty of ways to do the latter Thought Scour , any draw/discard effect ala Merfolk Looter . That could be another direction to take the deck.

You know what Sedris loves? His little pal Flayer of the Hatebound Unearth with him is such a delicious combo.You also get more from your mana if you have Mikaeus, the Unhallowed so your creatures are summoned, die, come back, die, then get Unearthed.

You could also use Ghostly Flicker to cancel out the Unearth effect and keep your creatures.