Looking at the interactions between:
Fog versus Undead Alchemist
Abuna's Chant versus Soul-Scar Mage

In each pair, the first represents an effect that prevents damage, while the second represents a replacement effect that does something instead of damage.

In either case, assume that the appropriate event has happened such that damage is on the verge of being done. Does the replacement effect occur, or does the continuous effect pre-empt the damage?

  • Fog and Abuna's Chant are both prevention effects, and the relevant abilities on Undead Alchemist and Soul-Scar Mage are both replacement effects. Your question doesn't actually seem to be about either continuous effects or triggered abilities. – murgatroid99 Oct 20 '17 at 17:44
  • @murgatroid99 Corrected the abilities, prevention effects are continuous effects, much like squares are rectangles. Now reading 616, though it is mildly tortuous. Suspect it might be APNAP, but still ruminating. – Drunk Cynic Oct 20 '17 at 17:53
  • Both prevention effects and replacement effects are types of continuous effects, but I'm sure you wouldn't write "Between continuous effects and continuous effects..." in your title. – murgatroid99 Oct 20 '17 at 17:55
  • @murgatroid99 Not saying you're wrong, just in the midst of editing, and wanted to make the math joke. – Drunk Cynic Oct 20 '17 at 17:58
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    Consider quoting the rules you found when you post. When you hover over the upvote button for a question, the tooltip says "This question shows research effort [...]". So you want to show research effort. – Rainbolt Oct 20 '17 at 19:23

Both prevention effects and replacement effects apply in the same way and at the same time, and the player who is affected or controls the object that is affected is the one who chooses the order in which to apply them. This is detailed in the rules section interaction of replacement and/or prevention effects:

  • 616.1. If two or more replacement and/or prevention effects are attempting to modify the way an event affects an object or player, the affected object’s controller (or its owner if it has no controller) or the affected player chooses one to apply, following the steps listed below. If two or more players have to make these choices at the same time, choices are made in APNAP order (see rule 101.4).

    • [...]

    • 616.1e Once the chosen effect has been applied, this process is repeated (taking into account only replacement or prevention effects that would now be applicable) until there are no more left to apply.

So, in both of your examples, the player that would be dealt damage decides which replacement effect to apply first.

In the Fog vs Undead Alchemist example, if a zombie controlled by Undead Alchemist's controller would deal combat damage to a player, that player chooses whether to apply the Fog effect and prevent the damage, or apply the Undead Alchemist effect first and mill cards. In either case, once one effect is applied the other one is no longer applicable.

In the Abuna's Chant vs Soul-Scar Mage example, once damage would be dealt by a source controlled by Soul-Scar Mage's controller to the creature that is the target of Abuna's Chant, the controller of that creature chooses which effect to apply first. If they choose to apply Soul-Scar Mage's ability first, the damage is replaced with -1/-1 counters, so the effect of Abuna's Chant is no longer applicable. If they choose to apply Abuna's Chant's effect first, then any remaining damage above the 5 it prevents is then replaced with -1/-1 counters.

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    This is also covered in Soul-scars Gatherer page: "If multiple prevention and/or replacement effects are trying to apply to the same damage, the controller of the creature that would be dealt damage chooses the order in which to apply them. As examples, an opponent could choose to apply the effect of Djeru’s Resolve and prevent the damage rather than put -1/-1 counters on it; or an opponent could choose to convert the damage to -1/-1 counters before Insult could double it, and then Insult’s effect won’t apply anymore." – Malco Oct 20 '17 at 18:07
  • With the edit, the first sentence of this answer is a bit weird; because it's saying what the question already says. – GendoIkari Oct 20 '17 at 18:44
  • I removed that sentence. – murgatroid99 Oct 20 '17 at 19:20

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