What are the cards label "offering" for?

Those cards are used specifically for Blood Brothers. They give 100xp to your skill.

They give 100xp to your character's skill if merged with it.

Use them in enhancing other cards under blood brothers. This increases the cards skill level.

Offering cards are use for solely to level up your other card skill level. They give quite a nice boost, 100xp, even better than a R rated card which only give you 90xp @ lvl 1.

They are not familiars, they are offerings. You can put them in your lineup, but they have no abilities. There purpose is only to be used as a blood brother when you combine familiars to increase the stats of your chosen familiar. The offerings give a 100 point bonus increase as apposed to the 9 that a common familiar would give.

Everyone has mentioned the most apparent aspect of the offering cards, but no one has mentioned that if you make the offerings blood brothers to each other you can get substantial point bonuses. Try combining 2 shades and then apply it to your familiar.

Do you mean combining them through evolution, or through blood brothers, because I know you can't evolve shades

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