Alessi is one of the Egypt 9 Glory Gods, controlling Sethan who can reduce the age of those who touches the shadowy Stand, even to the point of becoming a fetus.
It also de-ages the target's Stand, making it weaker and less potent.
At the climax of the battle, he managed to de-age Jotaro into a little boy and weaken his Stand. However, he was ultimately defeated when he underestimated Jotaro's weakened Stand, which was still comparatively powerful.