I know that Swain needs some nerfs,
but tripling the cooldown on his ultimate without any compensation is just too much.
If you really must do this, here are some ideas to keep him a decent champion (not all, just one. Or none, and skip a few lines to a less overboard nerf)
If you get a kill (either during the ult or while it is on CD), some of the cooldown is refunded. If you kill an enemy champion with Ravenous Flock active, the mana cost per second will reset to its initial value. If Ravenous Flock is active, killing a unit restores double mana from Carrion Renewal.
On the other hand, you could just nerf him in another way.
Make "Torment" only amp damage by 10% at rank 1, and make him need to scale it to get 20% damage amp.