Discarding one Spell Card is a Cost to activate the effect which Special Summons a monster from your Deck.[1]
If a monster like "Dark Ruler Ha Des" or "Revived King Ha Des" destroys a monster by battle, then the monster's effects are no longer negated if the monster leaves the Graveyard. Thus, if "Summoner Monk" is destroyed by battle with "Dark Ruler Ha Des" and is then Special Summoned from the Graveyard, then its effects are no longer negated, so it may Special Summon a Level 4 monster from the Deck.[2]