Concentration and attacks with special effects

Question

A Cleric that is concentrating on a Bless spell is hit by a Mummy Lord's Rotting Fist. The Cleric takes the damage (14 bludgeoning + 21 necrotic on average). Rotting Fist has a second effect:

If the target is a creature, it must succeed on a DC 16 Constitution saving throw or be cursed with mummy rot.

When does the Cleric make the concentration save in regards to the secondary effect of Rotting Fist?

• a) The Cleric makes the concentration save when they take the damage, meaning if they lose the concentration they do not add 1d4 to defend against mummy rot.

• b) The Cleric makes the concentration save after the entire attack is resolved, so they do get to add 1d4 to their mummy rot save regardless if they succeed on the concentration or not.

Which is the correct order?

Answer 1

The DM decides, optionally the player whose turn it is

The rule for concentration checks (PHB, p. 203) says:

Whenever you take damage while you are concentrating on a spell, you must make a Constitution saving throw to maintain your Concentration

so the Constitution saving throw to maintain concentration happens at the same time you take damage, not afterwards. This means all effects trigger simulaneously: the damage, the Constitution saving throw from the attack, and the Constitution saving throw from maintaining concentration.

The core rules are not fine grained enough to resolve the order here, so by default, this will be the DMs task to adjudicate.

Alternatively, the DM can use the optional rule from Xanathars Guide to Everything (p. 77):

If two or more things happen at the same time on a character or monster's turn, the person at the game table - whether player or DM - who controls that creature decides the order in which those things happen.

If you use this rule, and it is the player's turn, the player gets to decide.

Answer 2

Concentration rules say that whenever you take damage while you are concentrating on a spell, you must make a Constitution saving throw to maintain your Concentration.

However, the full effect of Rotting Fist as an action is both the damage and the saving throw. As such, I'd say those to effects happen simultaneously.

So it would be option B (The Cleric makes the concentration save after the entire attack is resolved, so they do get to add 1d4 to their mummy rot save regardless if they succeed on the concentration or not.)

Answer 3

Whoever has the turn, chooses (with optional rule)/DM's choice (without the optional rule)

As both effects are listed under the same "Hit:" block in the Mummy Lord's stat block and neither of them is explicitly happening before the other, it seems that they happen simultaneously. And if they do, then you may use the optional Simultaneous Effects rule from Xanathar's Guide to Everything:

If two or more things happen at the same time on a character or monster's turn, the person [...] who controls the creature decides the order in which these effects happen.

From that follows:

• If it is the Mummy Lord's turn, the DM chooses a) as an option giving the curse a best chance to stick
• If it is the Cleric's turn (say, if they are hit by an attack of opportunity), the Cleric's player chooses b) as an option giving him the best chance to resist the curse

If this rule is not in effect, then this choice probably falls to DM's discretion.