Learn Polish: Ta istota to rzekomo niewyjaśnione zjawisko. - This entity is allegedly an unexplained phenomenon.

While both deal with hearsay, rzekomo carries a stronger sense of skepticism. It translates closely to 'allegedly' or 'ostensibly' when you want to cast some doubt on the claim. If you used podobno, it would be a much more neutral report of what you have heard, similar to 'I heard that this entity is...'

Why is niewyjaśnione zjawisko in the nominative case here instead of the instrumental?

Because the sentence uses the word to as the linking verb. When you equate two noun phrases using to, both sides remain in the nominative case. If you wanted to use the actual verb być (to be), you would have to trigger the instrumental case for the second part: Ta istota jest rzekomo niewyjaśnionym zjawiskiem.

If I move rzekomo to the very beginning of the sentence, does it change the meaning?

Placing rzekomo at the beginning (Rzekomo ta istota...) frames the entire statement as an allegation right away. By placing it right before niewyjaśnione zjawisko as done here, the speaker specifically emphasizes that its status as an 'unexplained phenomenon' is the part being alleged. Both placements are grammatically correct and natural.

I have seen the word istota translated as 'essence'. How does it mean 'entity' here?