User blog comment:XCaidx/5* units for the new chapter/@comment-27177642-20160911094334/@comment-26944381-20160911150838

In the 3rd arc the summon units (Silas/Krantz/Felice/etc.) it's just the same as in normal quests, but your summoner unit (that is fixed as lead) need special LS spheres that are crafted on the town. So yes you can put Sacred blade to any of the units above, but not to your summoner who needs a special sphere.