The Curse of the Legendary Titans is an incredibly powerful and deadly curse that was placed on the Rings of the Legendary Titans by Lord Casterwill and that causes those who disturb them without the Amulet of Will to be struck by an incurable ailment. This ailment targets the very will of its victims and this cannot be combatted even through magic.
When the Ring of Araknos was disturbed by a young Simon Judeau, he was afflicted by the curse and hospitalized. source? In order to circumvent the curse, he made a pact with Araknos, binding their life forces together. source?
Later, when the Ring of Behemoth was disturbed by Metz, he similarly was afflicted and hospitalized under the care of Lady S.source? As the curse protecting the Ring of Behemoth was already activated, Dante Vale was able to use the ring without falling victim to the curse even though he wasn't wearing the Amulet of Will. S1E19