How many times per round does Hero's Beacon affect Tiamat?

Question

In the final encounter of Scales of War, each of Tiamat's heads gets its own initiative (on 45, 40, 35, 30, and 25) and can use one standard and one minor action. (If she wants to move, one head must turn its standard action into a move.)

30th-level bard cast Hero's Beacon on her:

Effect: the attack creates a zone of bright light in a burst 2 centered on the target. The zone lasts until the end of the encounter. When the target moves, the zone moves with it, remaining centered on it. Any ally who starts his or her turn within the zone regains hit points equal to your Charisma modifier. Any enemy that starts its turn within the zone takes 10 radiant damage.

Does this power deal 50 damage per round to Tiamat (10 per head/turn), or just 10 damage once per round?

There is a raging debate among my players.

Answer 1

Once

A “turn” for Tiamat encompasses the actions of all of her heads.

Tiamat’s description reads:

On each of her turns, she uses one or two breath weapons, trying to exhaust all of them so she can unleash her chromatic breath.

(Draconomicon, Chromatic Dragons, Tiamat, page 245.)

“Each turn” for Tiamat encompasses the actions of all her heads. (If each head got its own turn, then there would be no way to use two breath weapons on a given turn, as stated above.)

Her turn would begin on initiative count 45.

Also, Tiamat’s Frightful Majesty power ends on “Tiamat’s next turn” also indicating that it is Tiamat that gets a turn, not her heads.

The 6 Stat Blocks

There are 6 stat blocks for Tiamat: one for her and one for each of her heads. Only the “Tiamat” stat block lists Hit Points, so damage is dealt to Tiamat not to any particular head. So Tiamat’s Hit Points are reduced at the end of Tiamat’s turn.

The “White Dragon Head Only” stat block mentions “the end of this head’s next turn.”

We are left with a bit of a muddle: Tiamat has one turn a round, but each of her heads do as well.

But while a head can be said to have a turn, it has no Hit Point statistic to affect. Ongoing damage to Tiamat should occur on “Tiamat’s turn” — not the turns of her heads.

Contrast to the Ettin stat block

In contrast to Tiamat, an ettin (either Marauder or Spirit Talker) explicitly “gets two turns during a round, and has a full set of actions (standard, move, minor) on each turn. Each set of actions corresponds to a different head.” (Monster Manual, p.108).

While the ettin explicitly gets multiple turns per round, Tiamat explicitly gets a singular (albeit, multi-part) turn. Both of the ettin’s head gets a “turn” while each of Tiamat’s heads are called an “activation.”

The ettin’s ability to take immediate actions refreshes on each of its turns.

(MM, p 108)

Tiamat’s ability to take immediate actions refreshes on each head’s activation.

(Draconomicon, Chromatic Dragons, p. 245)

Answer 2

5 times

While Tiamat only receives one turn each round, that turn is not contiguous-- that is, it starts and stops multiple times with breaks in between. Things which would affect her once per turn still only happen once per round, but things that happen when her turn starts or ends happen multiple times, since her turn has multiple starting and ending points (specifically one each on counts 45, 40, 35, 30, and 25).

This is supported by the following PHB quote:

When your turn comes up in the initiative order, it’s time for you to act. Your turn has three parts: the start of your turn, the actions on your turn, and the end of your turn. (268, PHB)