You cannot activate "Ring of Destruction" during the Damage Step.
"Ring of Destruction" inflicts damage based on the current (total) ATK value, NOT the original ATK.
This card targets a monster.
If you chain "Ring of Destruction" to "Magic Cylinder", "Magic Cylinder"’s effect disappears because the monster is no longer there (so "Magic Cylinder" inflicts no damage).
If a player Tributes "Witch of the Black Forest" for "Summoned Skull", you can chain "Ring of Destruction" to the effect of "Witch of the Black Forest", in order to destroy "Summoned Skull". If this reduces the player’s Life Points to zero, then they cannot use "Witch of the Black Forest"’s effect (to retrieve a 5th Exodia piece, for example).