On this page of the Assassin's Creed Valhalla Crossover Stories guide you can find a walkthrough for Tossing and Turning story quest. During the mission, you'll meet with Kassandra - the hero of Assassin's Creed Odyssey.
Location of the cursed symbol: Cursed Church in the central part of the island.
You'll visit the location while in progress of the questline. Once you reach the area, a cut-scene will activate depicting a duel between Eivor and Kassandra. The cut-scene is rather lengthy, but it ends with an uneasy alliance between both parties.
Time to embark on a search for the cursed symbol. You need to deal with two destroyable walls. Use either oil jugs or Incendiary Powder Trap ability.
Escape and ensuing combat
Once the cursed symbol is dealt with, follow the cave to its exit. Simply move forward until you reach the exit.
There are enemies outside that have to be dealt with. Despite Kassandra's immense strength and abilities, she needs your help. Clear the area to end the quest at hand. Your next task is Night and Day - visit the walkthrough for this quest to continue.