This spell summons the dark brotherhood horse Shadowmere, this works even if you haven't got him yet or chose the destroy the Dark Brotherhood. I personally destroyed them and couldn't get the horse, however if this causes any bugs if you have the original Shadowmere please feel free to edit this mod if you know how otherwise leave bugs in the Posts Section.
You can find the Spell Tome on Farengar Secret-Fire's Table in Dragonreach or the table in the DB sanctuary (Falkreath one) where the Stone of Barenziah is or Unusual Stone is ( I added this after some user advise)!! SEE SCREENSHOTS!!

INSTALLATION

Install through NMM(Highly Recommended) or Manually by dragging the Summon able Shadowmere.esp into the Data Folder or just merge when you drag the Data folder from my Mod into the Skyrim Folder.

Uninstallation


Uninstall through NMM (Recommended) or delete the Shadowmere Summon.esp (If you do Uninstall I will personally shoot you in the knee!)