Je dirais le messager de la mort. Il y a la notion de précéder la mort et informer qu’elle arrive, le terme messager s’applique bien pour un personnage (alors que signe ou présage sont plus abstraits). Et ça sonne vraiment cool.

Je dirais “l’annonciateur de la mort”.

That’s the word that came most naturally to my mind, but if I had to analyze the motivations, I’d say

• it refers to a person, unlike a “signe avant-coureur”

• it’s in a similar language register as “harbinger” (literary)

• it goes well with “mort” because it’s often used with other dramatic predictions — eg “l’annonciateur de l’apocalypse”

Without discussing its standing as fine literature, considering the ancient back-story and fantasy-ridden context of The Transformers, I think the French version of “Harbinger of Death” as it’s to be used here could and should contain some literary and/or archaic flavor.
With this “criteria” in mind, regardless of whether one considers Megatron as a person or a thing, I think either précurseur or (in spite of you having already mentioned [& dismissed?] its English version, herald) the figurative sense of héraut could both be considered as distant “also rans” to Simon Déchamps’ good answer:

Précurseur
B. − Ce qui précède et annonce autre chose.
1. ….,

2. Être, chose ou événement présageant la venue d’une autre chose, d’un autre événement.

Héraut
B. − Au fig. Celui qui annonce la venue de quelqu’un ou de quelque chose ou qui en chante les louanges.
(both from CNRTL with emphasis added to the “ou” in the entry for “Héraut”)

Of these two, I would prefer
“le héraut de la mort,”
both for its added archaic flavor and because this n-gram seems to indicate that “le is used more than “un” with” héraut” (whereas “un” is used more than “le” with “précurseur”).
(please note that you may have to re-click the blue search button to get the results)

(but see this n-gram with “un/le messager” and “de la mort” added to the search, which probably serves only to confirm Simon’s answer)