Sedna has low HP because she doesn't take BotF, everyone piles on sedna, sedna has to keep lowering her health to heal herself, Sedna dies. The -10% health would really hurt, but it would help save allies every 30-45 seconds 
This item would be great for queen because her monks could heal off the -10% while her shield is up, and it will give her the mana for an extra two shields or to summon some shamblers.
Sedna's heal has a fairly long cooldown, so she won't be able to heal herself as efficiently as queen can shield using this item. It woud still be pretty good. It would also be good with DA and rook.