One Light in the Darkness (25 player)

Prior to the fight you can speak to each of the four Watchers to activate them for the encounter. Each one gives a buff to the entire raid.

Freya -- Resilience of Nature Increases all damage done by 10% and healing received from all sources by 20%.
Mimiron -- Speed of Invention Increases all damage done by 10% and movement speed by 20%.
Thorim -- Fury of the Storm Increases all damage done by 10% and total health by 20%.
Hodir -- Fortitude of Frost Increases damage done by 10% and reduces damage taken from all sources by 20%.

In addition, each of the Watchers also adds a unique ability to the fight.

Freya -- Sanity Well Standing in the well brings you back to your senses, regenerating 20% Sanity every 2 sec. Damage done is reduced by 50% while you remain in the well. They will spawn around the room and this is the only way to regain Sanity during the fight.
Mimiron -- Will cast Destabilization Matrix on the tentacles that spawn during the second phase of the encounter. This allows you to safely ignore a majority of these adds during the fight.
Thorim -- Will cast Titanic Storm during Phase 3 of the encounter. This is the only way to kill the adds that spawn during Phase 3. Otherwise they will simply remain at 0% health.
Hodir -- Hodir's Protective Gaze Allows Hodir to save a friendly target who sustains fatal damage by encasing them in a nearly impenetrable block of ice for up to until cancelled. Hodir must rest after performing this action.

For this achievement, you may only activate one of the Watchers for the fight. Choose wisely.

Also note that the true Hard Mode for Yogg-Saron is to do this without the assistance of any Watchers.

Actually, this is considered true enough the real hardmode. You will get hardmode loot for this.

While Mimiron and Hodir merely offer a significant bonus, without Thorim, there is no way to kill the Immortal Guardians that spawn during phase 3, which means they must be tanked til Yogg dies.

Then again, not having Freya's Sanity Wells is a huge problem on its own, seeing as it's the only way for Sanity to recover in the fight.

Do note that this fight was made while considering the possibility of having no keepers at all, so it seems like this specific last choice seems like a personal guild preference.