Dark Consumption should probably cost 1BB.
Keelhaul should read: "Gain control of target creature an opponent controls. Untap that creature. It gains haste. At the end of your turn, sacrifice it." It should also be a sorcery, seeing as you're making the sacrifice at the end of YOUR turn. If you want it to be an instant speed, the last line should read "At the beginning of the next end step, sacrifice it." Either way, seeing as this is an effect that's "better" than Act of Treason, it should cost more, 1UU at the least. Seeing as it's a red effect in blue, you might be able to argue it should be UUU or 2UU.
The second ability of Ancient Temple should be formatted "{t}, {2}{G}:"
Nature's Serenity should be an Enchantment- Aura with first line "Enchant creature."
Ent is not a creature type. You're looking for Treefolk. Beyond that, for 5G, you should be able to get a 6/6 vanilla. Anything less (without buff) is criminal, and upping the costs (5G is easier to get than 4GG) just drops it way below curve. If you're looking for a 5/6 vanilla, perhaps 3GG is an appropriate cost?