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 you can purchase it from most spell vendors. And remember all of the files are to be used separately NOT TOGETHER, the spell tome is then found on the table 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!)

P.S All versions apart from Version 1.0 the book is found in Astrids Room in the Original Dark Brotherhood Sanctuary (NOT DAWNSTAR) If you want me to add it in Dwanstar please say but otherwise the one place is enough as it was Astrids horse and it seems fitting its in her room.