Why "more severe are" is put in front of "its drawbacks"?

The higher your Vampire stage is, the stronger you become, but also more severe are its drawbacks.

This would be better as

The higher your Vampire stage is, the stronger you become, but also the more severe are its drawbacks.

This follows the pattern of

The + comparative + subject + verb, the + comparative + subject + verb. (The older we grow, the wiser we become.)

The construction is often followed by (i) a conclusion or (ii) contrast in the same form

(i) The older we grow, the wiser we become and the happier is our life.

(ii) The older we grow, the wiser we become but the less time we have. (You can also say "but the less time have we" as a form of parallelism.

(Have a look at the "Grammar Org" website for "The ... the.. " constructions)

The inversion is stylistic. You can begin a sentence with an adverb/adverbial or adjective/adjectival if you wish to add emphasis. If you do this then you could also invert the verb and subject:

"Hot(adj.) was the day, long(adj.) was the road, and dearly(adv.) did he wish to rest."

About 300 years ago, this sort of construction was relatively common, especially in literature. It has become less common in current Modern English but is sometimes found in particular constructions - "the...the" is one of them.