I didn't know for sure wut to put his original ATK and DEF at so it may be a little overpowered and I threw in Nightmare Horse's effect but I put a cost with that
if u can't read his effect it's right here:
"Headless Knight" + "Nightmare Horse"
This card can not be Special Summoned except by Fusion Summon. This card gains 500 ATK and DEF for each Zombie-Type monster on the field and in your graveyard. This card can attack your opponents life points directly but you must Tribute a Zombie-Type monster to activate this effect. When this card is destroyed and sent to the graveyard you can Special Summon 1 Zombie-Type monster from your graveyard.
if u can't read his effect it's right here:
"Headless Knight" + "Nightmare Horse"
This card can not be Special Summoned except by Fusion Summon. This card gains 500 ATK and DEF for each Zombie-Type monster on the field and in your graveyard. This card can attack your opponents life points directly but you must Tribute a Zombie-Type monster to activate this effect. When this card is destroyed and sent to the graveyard you can Special Summon 1 Zombie-Type monster from your graveyard.