While “Dimensional Fissure” is active, a monster that is destroyed while face-down will still be removed from play face-up.
Since Monster Cards are removed from play instead of going to the Graveyard, you cannot send “Horus the Black Flame Dragon LV6” to the Graveyard to activate its effect to level up.
If you also have "Gravekeeper's Servant" active on the field, your opponent cannot declare an attack.