When they make card battle RPGs, usually it's a character/monster a card, with hundreds of individual artists / designers that each work on different cards -- exactly the sort of thing you would expect with a listing out of a bunch of established artist names, as included within this trailer. But obviously, the seven "Grails" may be something else.
To put it in context, arcade-based card battle games like Sangokushi Taisen frequently have "Commander" cards which are not directly playable in combat, and "Officer" cards that represent named unit leaders -- usually legendary warriors. The "Commander" is a leader character who levels up (yes, per-card-stats are saved to your arcade machine memory card) on the basis of overall battle outcomes, while individual "Officers" level up based on their victories and losses during a campaign.
Assuming that a similar system applies here, the seven "Grails" might be something like "Ruler" type team leaders that each summon teams of servants during combat.
With such a system in place, we could possibly have a Fate / Team Fortress in earnest.