AJ Christiaens
Mortis likes dead things, especially throwing those dead things at his opponents. Mortis Grimm Midrange is a deck that uses the discard pile like no other. Just flip your deck upside down, and get all the value.
To get his deadly engine going there are some notable cards you want to see early. Gifts From The Grave being the most useful early game start, though it falls off hard past the first couple of turns. Sir Grimm Crimson, The Undead Prince on the other hand is always good, and he helps the dead keep walking with endless card advantage. Having 1-2 revive effects in hand like Windmill Of Corpses or Across The River Styx to start, allows him to keep giving you want you need.
Early Tips & Playlines
The deck is mostly broken into three parts: cards that mill, cards that do things when milled or in discard, and Warriors. your first turn you want to start milling and using this to gain access to more of your deck. The deck is largely left to its own demise, relying on more luck then others by milling but has many cards that fill the same role to help mediate this.
Besides just throwing your deck into the discard pile, its always good to draw cards too. Grimmace, The Grimm Princess and Agatha, Ancient Girl Of The Grimm being revived a few times helps keep your hand full.
Prysm, Soul-Collector Of The Grimm and Esmeralda, The Grimm Necromancer not only bring their dead friends back but each other. This allows you to loop your undead army until you can't kill them anymore.
Late Tips & Playlines
Generally once you have the set up and the opening, you want to use Slow But Grimm Decay to finish off your opponent. Mortis' Special Ability can help clear your board to add some extra damage or you can try to stick a Spores From The Suffering Cyprus to finish the job.
If you find you don't have the ability to push to much damage it might be best to make sure you have the Prysm and Esmeralda loop or ways to restart it, with Gone But Never Forgotten being the best option to do so.
you will find that your deck gets flipped pretty quick once the ball gets rolling. Try to be aware of what you have access to and what you might not be able to use with no cards in deck. That being said, with no deck and Across The River Styx plus Sir Grimm Crimson, The Undead Prince you can set up pretty much any play you need.