Discard "D/D Necro Slime" with this card's effect, then use the effect of "Necro Slime" to easily get 2 Fusion Summons.
Use this card with "D/D Lamia" to summon a Level 6 or 7 "D/D/D" Fusion Monster, then use "Lamia"'s effect to special summon itself from the graveyard (potentially by discarding "Necro Slime") and tune it with the fusion monster to synchro summon a Level 7 or 8 "D/D/D" Synchro Monster.
Depending on what Extra Deck monsters you summon in this combo, you can potentially revive whatever lower-level fusion monster you used.
This card is a principal component in many useful combos which revolve around combinations of itself, "D/D Lamia", and "D/D Necro Slime" to successively summon a number of potent "D/D/D" Extra Deck monsters in a single turn. The most significant combos are explained here.