Its an ad, it feels very dark. Seems pretty relevant to the subject matter.
Well sure it's a bit "dark", but more importantly it does carry the Herakles symbolism.
It roughly means "you may die in your infancy, but you can also endure all the difficulties and become a hero, founder of a dynasty!", while also being reminiscent on one of the iconic (but not also "realistic") assassinations in CK2.
Snakes and snake bites carry a very, very heavy symbolism. Snakes are the devil/Satan in the middle ages, they are also a death of choice for various famous people (like Cleopatra), they are also an execution method that was common in many parts of the world. The trailer shows the struggle against evil, it's also easily interpreted as an assassination attempt against a member of an important dynasty - and maybe an attempt at destroying a dynasty. But it's also a manifestation of the test of time.
The trailer also carry the new "velvet-y", elegant graphic design and atmosphere of CK3 compared to CK2. The trailers for CK2 were fun and goofy, already full of meme potential. CK3 seems modernized, more "serious" in a way. The world of CK3 is unforgiving, and not as light-hearted. I think it's an important message too.