The word "immediatly" is explicitly in the text. It does not arise from the rendering into English of some Greek verb tense.
The E-sword program gives the following for the word translated as "immediatly" in Mark 1:12.
εὐθύς
euthus
yoo-thoos'
Perhaps from G2095 and G5087; straight, that is, (literally) level, or (figuratively) true; adverbially (of time) at once: - anon, by and by, forthwith, immediately, straightway.