How do you summon Horus the Black Flame Dragon Lv8?

When Horus Lv6 destroys a monster by battle, you may send it to the Graveyard it to special summon Horus Lv8 in your end phase.

Or, the card Level Up!! can bring him out too. Even though Lv6 is unaffected by spell cards, you are sending it to the Graveyard as a Cost, so you can still do it.