Blood Shower was a spell acquired and used by Kain during the events of Blood Omen: Legacy of Kain. Kain found it during his visit to Vorador's Mansion. This spell allowed him to drain and feed with the blood of up to four nearby enemies, while the other victims that were in the vicinity would suddenly die nonetheless. Also, Kain would had to avoid using this spell around creatures with tainted (black/green colored) blood, that could surely poison or harm him. Also, bosses are unaffected by this spell.
Like many other of Kain spells, Blood Shower was only used in Blood Omen: Legacy of Kain.
Like many of Kain's Blood Omen abilities the Blood Shower spell is not seen in further games. Legacy of Kain: Defiance allowed Kain to use telekinesis to drink blood from from multiple opponents (providing they were all in the waver state), arguably providing a similar visual effect to the Blood Shower spell. However this ability notably lacks Blood Shower's reliance on Magic energy (itself not seen after Blood Omen) and doesn't kill all on-screen opponents.