5% max hp on the passive proc for her Ult is pretty low. Think about making it 10%, or 15% and removing the attack speed buff. A sealing circle doesn't really lend itself to buffs as much as debuffs.
20% mana is still a bit much. Perhaps have it scale off of Base Mana instead of Total Mana? Or rework the wording and give flat energy, flat rage, and % base mana?
Or were you removing the mana from the effect?
Also, you don't necessarily have to make the stun half duration. You can make it 1.5 sec stun at all ranks and reduce the cooldown by a few seconds. 19 sec at rank one is pretty long compared to Soraka's Infuse (10s cd at all ranks) which has a similar effect.