"Die Hexe hat den Prinzen in einen Drachen verwandelt."

Translation: The witch transformed the prince into a dragon.

[StanczakDominik]

That... Really doesn't sound like a good strategy.

[D.Krynicki]

Why not: "the witch has transformed the prince into a dragon"?

[taktmann]

Why not einem?

[Vabelie]

As there is a transformation, here, I would definitively lean toward accusative, not dative. Not sure though if it's the correct explaination.

[shinarit]

Why not "turned"?

[RoniH80]

Why den Prinzen? Is this somehow Dativ?.. Why?

[Vabelie]

Der Prinz is a masculine weak noun See the linked pages for a few explanations

http://www.vistawide.com/german/grammar/german_nouns03.htm

http://germanforenglishspeakers.com/nouns/weak-nouns-the-n-declension/

[mindgrain]

'den Prinzen' is accusative: change him into sthg.

[adinsh]

Couldn't it be accusative plural? She transforms the princes into dragons

[mindgrain]

No, because you have 'einen' there... so it is 'into a', definitely singular.

[adinsh]

Transformed

[MariaReitz]

Why not: "The witch has the prince transformed to a dragon?"

[NeoTubNinja]

I couldn't explain why, but into is definitely the appropriate preposition to use after transformed.

In addition to that, "The witch has the prince transformed into a dragon.", while grammatically correct in English, kind of implies that she didn't directly transform the prince herself. Maybe it was a potion she brewed. Maybe she read a magic scroll. Each of those objects would have done the transforming in my mind. If she had done it herself, I'd definitely say "The witch has transformed the prince into a dragon." would be more commonly used.