Unlike another question that asks about the vague prophecy Rivkah was given about her twins, I ask specifically about Itzhak's statement to Esav:
... וְאֶת־אָחִיךָ תַּעֲבֹד ...׃ - And you shall serve your brother; Genesis.27.40
As I can see, most commentators skip this part of Isaac's prophecy. Unlike the other question, this verse has a proper grammatical form "את אחיך" similar to "אֶת ה' אֱלֹהֶיךָ תִּירָא אֹתוֹ תַעֲבֹד", meaning serving and worshipping, this verse states clearly that Esav should subsere to Yaakov.
Was it really a blessing or a prophecy? If a blessing, what kind of blessing is that? If a prophecy, what does this mean for Esav and when was it fulfilled?