Why is it der Magier?

Magier ends in -er, and most nouns ending in -er are masculine: in a German dictionary, 86% of them take der.

All nouns ending in -er
Sense 1
der

Meaning

[1] person who has supernatural skills and uses

[1] Person, die übernatürliche Fähigkeiten besitzt und anwendet

[2] Person who demonstrates magic tricks

[2] Person, die Zaubertricks vorführt

Examples

[1] "The magician waited silently until the man had left the hall"

[1] „Der Magier wartete still, bis der Mann den Saal verlassen hatte.“

[2] The magician conjured up a rabbit out of his cylinder

[2] Der Magier zauberte ein Kaninchen aus seinem Zylinder hervor.
More source examples (1)

These dictionary examples can include historical and less common uses.

[2] "This one had something of the magicians who attracted tents at the fair in the fairs, but the charlatanism was missing"

[2] „Dieser hier hatte etwas von den Magiern, die auf den Jahrmärkten zum Eintritt in verhangene Zelte anlocken, doch fehlte ihm die Scharlatanerie.“

Declension

1 Singular Plural
Nominativ der Magier die Magier
Genitiv des Magiers der Magier
Dativ dem Magier den Magiern
Akkusativ den Magier die Magier

Pronunciation

Dictionary entries include material from Wiktionary. Wiktionary