To acquire the summoning spell, venturing into dunegeon that was once sacrosanct, but now a derelict catatomb-"Sancre Tor" to quest five fragments of the "Sinister Core", all of which are embraced in Ayleid chest known as "Sinister Coffer", which are well concealed within the ample Sancre Tor.
Upon gathering all five pieces, they'll automatically reform the central core-"Sinister Core", and you'll gain the summoning spell-"Sinister Wrath".
P.S If the spell don't appear in list automatically, drop down the "Sinister Core" and pick it up again.