Eternal Decks · Board game rules
do you have to cover skull cards in the river for stage F
Quick answer
No — Skull Cards are never "covered" in the River; they can only be played to the River at all after you've gained the Key Bonus Rare Card, and then they're simply used in River recipes as-is.
Details
Under the general Skull Card rules, "Skull Cards cannot be played to the River" [p.36 — Additional notes about Skull Cards]. This restriction still applies in Stage F unless it's been lifted.
In Stage F specifically, once a player gains the Key Bonus Rare Card, it is immediately played to the leftmost space of the River (moving any card already there to another empty space) [p.44 — Generating Jewels from the River]. From that point forward, "Skull Cards in your hand can now be played to the River. This also means Skull Cards can be used to generate River recipes" [p.44 — Generating Jewels from the River]. The Key Bonus Rare Card and matching-colored Skull Cards can also help fulfill any of the four River recipes, not just the two that explicitly require a Rare/Skull [p.44 — Generating Jewels from the River].
There is no mechanic in the River where a Number Card must be played "on top of" a Skull Card the way it works in the Field (Stage D) [p.36 — Playing a Number Card on top of a Skull Card]. That "covering" concept only applies to Skull Cards played to the Field, not the River.
Watch out: Before you've gained the Key Bonus Rare Card in Stage F, Skull Cards simply cannot be played to the River at all — there's no way to "cover" them there either. Only after that specific Key Bonus is obtained does the River accept Skull Cards, and they participate freely in Jewel recipes without needing to be covered by anything.