This sounds like:

Reserve Untapped by SilverEyedHyuuga
Years of battling Naraku have kindled Kagome's formerly feeble reiki into a considerable force. Now that the spider demon has been defeated, she has taken it upon herself to train and maintain her power. A certain daiyoukai cannot help but be intrigued by the blossoming of her gifts, and when their chance encounters begin to become not so coincidental anymore, he, against his better judgement and opinion of humans, begins to recognize her for the formidable priestess she has become. When Sesshoumaru joins forces with Kagome against a deadly new enemy--the powers of which they have never encountered before--he comes to respect her more and more as not just a mere miko, but as his equal.

This is rated MA!

The part you describe sounds like Chapter 17-18. Hope it's what you're looking for!

<3 L

Hi everyone!
I've been trying to find a story for the past couple if days and it's killing me that I can't find it for some reason!

All I remember is that it is post Naraku and there is another war going on and the enemy is very powerful and using a demon/puppet army. Sesshoumaru ends up transferring part of himself to Kagome so the enemy cannot use his full power agains them. Unfortunately during the final battle Sesshoumaru ends up needing to go into his true form and ends up being captured. After being tortured, he ultimately ends up dying from his severe wounds. When Kagome ends up finding where the enemy is she is able to defeat him by using the decoy of Sesshoumaru that the enemy made against him since she is the one who holds Sesshoumaru's power.
The real Sesshoumaru has died but since Kagome is bad ass she brings him back to life and they live happily ever after (of course).

Thank you so much!

Yes! You are right, that's the one!
Oh you are seriously the bomb diggity.
Thank you so much!