Is it present on all maps?
Of course not. It depends on the map, but if the map has an artifact shop or mage and Town Portal castles, the chances of finding it increase. It makes a lot of sense to look for and use the amulet when playing against a single opponent, as having another type of troop in the cursed ones never hurts. I would also suggest that the developers make it so that the Ruthless Avenger not only adds another enemy but also increases the chance of a critical hit. Let's say by another 10%. Then it would be 60%. But even with 50%, it works well.