Learn German: Eine Prise dieses magischen Krautes wird schnell gefährlich. - A pinch of this magical herb quickly becomes dangerous.

While we normally use apposition for quantities (like eine Prise Salz without a case change), the rule changes if the measured noun includes a definite article, a possessive, or a demonstrative word like dieses. In those cases, you must use the partitive genitive. So, 'a pinch of magical herb' could be eine Prise magisches Kraut, but 'a pinch of this magical herb' requires the genitive: eine Prise dieses magischen Krautes.

Why does magischen end in -en?

It follows the demonstrative dieses, which already does the heavy lifting of showing the neuter genitive case with its -es ending. Therefore, the adjective magisch takes the weak declension ending for the genitive, which is -en.

Why does the noun Kraut get an -es at the end?

Kraut (herb) is a neuter noun (das Kraut). In the genitive singular, masculine and neuter nouns add either -s or -es. For short, one-syllable words like Kraut, adding -es is the traditional standard and often sounds more natural, though Krauts is also grammatically acceptable.

Is werden always used for 'becomes'? I still mix it up with bekommen.