User blog comment:ShadowEyepatch/Shadow Deities/@comment-7341673-20160504130647/@comment-7341673-20160504142347

Then could either have them placed under rearguards face-up or just have cards other than the cards themselves use the a skill to call them (ex: CB 1 & choose one of your rear guards with a card underneath it and retire it. When this unit attacks, you may pay the cost. If you do, choose a card underneath a unit retired by this effect and call it to RC).

That would reduce Shadow to just "place this card underneath a rear guard" and nothing more.