[quote=@The World]
[@Double]
For a Horus deck, it looks like you're forgoing a lot of its [url=https://www.db.yugioh-card.com/yugiohdb/card_search.action?ope=2&cid=6100]support cards[/url], if those interest you at all. As for a new custom card... Maybe a special card? 
- Since there's only one custom card per player, a Ritual is out. 
- Fusion would be a little weird for a deck based around a single monster leveling up. 
- Synchro would need a tuner, but since lvl 10+ is god territory anyway, maybe a level 10 synchro? I'll come back to this in a moment.
- XYZ would need multiple level 8s, which without going through your whole deck list, I don't know how plausible that is.
- Pendulum is useless in a deck with only one pendulum monster.
- Link maybe? A link 3 or 4 Horus that requires Horus Lvl 8 to be one of its chain links? That's a little more complex to come up with than the other kinds, though.

Maybe something like: 

[hider=Card Idea]
"Horus the Majestic Black Flame"
Level 10 Synchro Monster
"Majestic Dragon" + 1+ more tuner monsters + "Horus the Black Flame Dragon"
Must first be Synchro Summoned. While this card is on the field, your opponent cannot activate Spell cards or their effects without first tributing 1 monster from their hand or field. Once per turn, you may destroy one Spell/Trap card on the field, and if you do, banish it. If this card destroys an opponent's monster by battle and sends it to the GY, inflict damage to your opponent's LP equal to the ATK of that destroyed monster.
[/hider]
Then it's just throw in Majestic Dragon, and 1+ Tuners such as Rider of the Storm Winds to buff out the possibility of sychroing with lower than level 8 Horus.
[/quote]

It's not an actual Horus deck. It's just a dragon structure deck that has Horus splashed into it. That's why it lacks the LV support cards.