Bright Sword

This magic sword casts a Flaming Sword of Ruin spell at the beginning of each player's turn (the opponent cannot stop this at all). The spell can then be dispelled normally in each player's Magic phase, but that just temporarily quenches its fire and the sword automatically burst into new flames at the beginning of the next turn.
Note that Thyrus cannot cast the Flaming Sword on himself (there are no cumulative effects).