The words how and as can function as adverbial relative pronouns with verbs like call.
The verb call can be used in different ways:
With an object complement denoting the name you assign:
He called her his mistress.
With an adverbial constituent of manner denoting how you call something, often similar to assigning a name:
Her called her by her last name.
The words how and as (similar to the adverbial phrase the way [that]) can replace the adverbial phrase and function as adverbial relative pronouns, just like where:
He is here. (adverb / adverbial demonstrative pronoun of place)
Where is he? (adverbial interrogative pronoun)
Mordor is where the shadows lie. (adverbial relative pronoun, defining relative clause)
He is in Mordor, where the shadows lie. (adverbial relative pronoun, non-defining)
With so/how/as:
So is Gondor. (adverb / adverbial demonstrative pronoun of manner)
How is Gondor? (adverbial interrogative pronoun)
Gondor is how I imagined it. (adverbial relative pronoun, defining)
Gondor is as it should be. (adverbial relative pronoun, defining)
She opposes Sauron, as she should do. (adverbial relative pronoun, non-defining)
With verbs meaning "address or assign a name to":
They named Mordor so because of its dark shadows: mor means "dark", dor "land".
They called it thus. (adverb / adverbial demonstrative pronoun of manner)
?They called Mordor so.
?How would you call a land like Mordor but in present-day Asia?
?That's not as they called it in the Silmarillion. (adverbial relative pronoun, defining)
Mordor, or 'the land where the shadows lie', as they called it, was well defended. (non-defining)
With the word call, the construction with an object complement (1) is common and always possible. The construction with an adverbial constituent (2), however, is only common with normal adverbial phrases, with the adverbial demonstrative pronoun thus, and with as used as an adverbial relative pronoun introducing a non-defining relative clause, as in the last example and also in your example. I don't have an explanation for this: language is fickle.
Because object complements (1) and adverbial phrases (2) are often interchangeable with verbs like call, they tend to blur together sometimes. In non-standard English, you will even find as used as a simple subject or object relative pronoun:
%The critter as did this was a big one. ("The critter that did this was a big one.")
