how to use the Inferno faction's "Sacrifice" ability?

without a sacrificial pit, you won't be able to build a castle, and that's not very good when attacking – so it's worth building it either as soon as the main buildings for troops are available, or earlier, when you urgently need to prepare for defense during an attack...

As for horned demons, I would strongly advise against using them as "sacrifices"! They usually participate little in battles, especially with neutrals, due to their low speed – which means that closer to the supposed end of the game – the battle with the main opponent, there will be a very, very decent number of them... It was precisely due to their numbers that I was able to win difficult battles more than once...