Spoiler monolith endgame?

[Kasa]

I have never actually made it that far into the DLC.

With the new update if I chose horror mode what a I losing for this monolith endgame event?

[HunterSilver]

So this shouldn't come as too much of a surprise, but you'll get waves of raids if you get to the endgame for Anomaly. I know, shocker, truly no other ending in RimWorld has done this.

Once you fully activate the monolith:

Two obelisks that are corrupting your world will spawn in along with several simultaneous anomaly raids and a constant flow of anomaly units. Activating the obelisks progresses the ending but each one spawns several raids once activated.

After activating both, unnatural darkness and more raids will spawn along with 3 more obelisks eventually. You don't need to kill all of these entities, you just need to activate the 3 obelisks before something weird happens with your monolith and it summons a horde of entities around itself. Again, you don't have to kill these, just get someone to the monolith and keep them alive as they activate it for the final time. Whoever is activating it will be sent through a void portal for a final decision.

Once you've entered the void portal you're given a choice: You can reset the monolith back to its first form or you can become one with the void entity and keep the anomalies in your world permanently. If you become one with the void entity your colonist gets a massive permanent buff and no longer needs sleep or recreation, gains ghoul regeneration (100hp per day per body part, including scarred and lost limbs and organs), +100% psychic sensitivity, and +100% entity study speed). Their hair and eyes also become pure white.

[Kasa]

So a neat little thing but probably not ultimately important.
YMMV.