At the end of its turn, this champion moves 2 spaces toward the closest enemy champion, and makes an attack against that champion if possible.
The attack from manic isn't a basic attack, meaning that you cannot target impenetrable units with it, the champion will move normally, but if it ends engaged, it won't attack.
To predict the movement first you have to know the ability will target the nearest champion to move, if there are two at the same distance, then manic will target the champion that was deployed earlier. If you aren't in a straight line, the champion will try to move first all he can in the first direction counting counterclockwise until he's in line with target champion. When targeting 2x2 champions, the upper left square of the champion is what is targeted as it was a 1x1 champion.