Learn Spanish (Latin America): El unicornio mágico brilla como si fuera inmortal. - The magic unicorn shines as if it were immortal.

The phrase como si (as if) sets up a hypothetical or imaginary comparison. In Spanish, when you make a hypothetical comparison about the present, como si must always be followed by the imperfect subjunctive. Even though the shining is happening now, the idea of the unicorn being immortal is contrary-to-fact, so we use fuera instead of the present subjunctive sea.

Is fuera coming from the verb ser (to be) or ir (to go) in this sentence?

It comes from ser. The verbs ser and ir share the exact same forms in the imperfect subjunctive (just like they do in the preterite past tense). Context tells us which one it is: since the sentence links the subject to a descriptive adjective (inmortal), it translates to "as if it were immortal" rather than "as if it went."

Could I say como si fuese inmortal instead?

Yes, absolutely. The imperfect subjunctive has two sets of interchangeable endings: -ra (like fuera) and -se (like fuese). Both are grammatically correct and mean the exact same thing. However, in Latin America, you will hear the -ra forms much more frequently in everyday speech.

Why does mágico have a written accent mark on the first 'a'?