Whether the targeted permanent or player is still a legal target is not checked after Divine Deflection resolves. For example, if a creature targeted by Divine Deflection gains shroud after Divine Deflection resolves, Divine Deflection can still deal damage to that creature.

If Divine Deflection prevents damage, excess damage (if any) dealt by that source is dealt at the same time. Immediately afterward, as part of that same prevention effect, Divine Deflection deals its damage. This happens before state-based actions are checked, and before the spell or ability that caused damage to be dealt resumes its resolution.

If Divine Deflection can’t deal damage to the targeted permanent or player (because the creature is no longer on the battlefield, or is no longer a creature, or the player is no longer in the game, for example), it will still prevent damage. It just won’t deal any damage itself.

If the targeted permanent or player is an illegal target by the time Divine Deflection resolves, the entire spell doesn’t resolve. No damage will be prevented.

If damage is dealt to multiple permanents you control, or is dealt to you and at least one permanent you control, you choose which of that damage to prevent if the chosen value for X won’t prevent all the damage. For example, if 3 damage would be dealt to you and to each of two creatures you control, and Divine Deflection will prevent the next 3 damage, you might choose to prevent the next 2 damage it would deal to you and the next 1 damage it would deal to one of the creatures, among other choices. You don’t decide until the point at which the damage would be dealt.

Divine Deflection’s effect is not a redirection effect. If it prevents damage, Divine Deflection (not the original source) deals damage to the targeted permanent or player as part of that prevention effect. Divine Deflection is the source of the new damage, so the characteristics of the original source (such as its color, or whether it had lifelink or deathtouch) don’t affect this damage. The new damage is not combat damage, even if the prevented damage was.

Divine Deflection can prevent damage that would be dealt to you, one or more creatures you control, and/or one or more planeswalkers you control.

Divine Deflection’s only target is the permanent or player it may deal damage to. You choose that target as you cast Divine Deflection, not at the time it prevents damage.

Divine Deflection can prevent damage from multiple sources dealing damage simultaneously (such as during combat). If the chosen value for X won’t prevent all the damage, you choose which sources to prevent damage from.