Certainly We took a covenant from the Children of Israel and sent them messengers. Whenever there came to them a messenger with a message that did not suit their fancies, either they called him an impostor or they put him to death.
Meaning:-
Covenant: contract, agreement, undertaking, commitment, guarantee, pledge, promise.
Fancies: imagination, believe or fantasy, a mental image or conception:
Impostor: a person who pretends to be someone else in order to deceive others, especially for fraudulent gain.