The 9 counter debuff implies that it's a hard time limit for the fight.
The stake that got Tsubaki is...not new.
It's actually a gimmick shown before, and Hnagiku figured out after she recalled similar nail/cursed doll from all the way of their last visit to the mountain:
https://mangadex.org/chapter/044aef82-4eea-4625-bd08-ff415b245617/12
Boss stake half the people, and to break out you must break the cursed dolls on its horn.
It can also make copy of the staked people to fight remaining people.