Jeremy Crawford esclareceu em um tweet que Fey Ancestry só protege contra o sono mágico.

Nonmagical sleep gas/poison/etc. can knock an elf unconscious, unless its description says otherwise.

Ele também esclareceu outro tweet que a respiração de um Dragão não é mágica.

The breath weapon of a typical dragon is not magical.

A menos que seja explicitamente declarado no texto da Tumba da Aniquilação que a respiração deste Dragão em particular é mágica, o efeito do gás do sono não seria impedido pela Ancestrais das Fey.

Como mencionado por V2Blast, o Compêndio de Aconselhamento de Sage responde à pergunta "É o sopro de um dragão mágico? como abaixo (ênfase minha):

Determining whether a game feature is magical is straightforward. Ask yourself these questions about the feature:

• Is it a magic item?
• Is it a spell? Or does it let you create the effects of a spell that’s mentioned in its description?
• Is it a spell attack?
• Is it fueled by the use of spell slots?
• Does its description say it’s magical?

If your answer to any of those questions is yes, the feature is magical.

Let’s look at a white dragon’s Cold Breath and ask ourselves those questions. First, Cold Breath isn’t a magic item. Second, its description mentions no spell. Third, it’s not a spell attack. Fourth, the word “magical” appears nowhere in its description. Our conclusion: Cold Breath is not considered a magical game effect, even though we know that dragons are amazing, supernatural beings.