🍁Nine Ancient Obelisks

How exactly is this spell "applied"? Out loud and "for all to hear" or "mentally and discreetly"?
Do you need to enter the "room" or can the spell be cast "nearby, on the street"?
Does this spell require OD?

Mentally, but visually noticeable.
You can cast it from the outside.
It requires OD.

Added 20 minutes ago

- Oh! Hey! - pushing through the pre-plague feast, the crab greets his comrade. - Let's hang out.

- Hang out?... - Hisoka, who greeted them last, didn't understand. - Your vocabulary... As always, it's very rich and exotic...
Over the past few months, Sunaro has hardly seen Akimitsu and Kami, spending most of his time with Hisoka, Zumi, and Nemuyasu, so today was a rare opportunity to see his brother and Kami again, with whom he hasn't yet become as close as with the others. Apparently, Yuami, the granddaughter of the guild leader, was in charge of his training.
- Yeah, well, that's the news! - After the greetings were exchanged, he steered the conversation towards a current topic. - It seems you made the right career choice, Akimitsu; you'll now be our mainstay in the fight against the undead.
- Exactly! - A self-satisfied smirk adorned the bandit's face. - I never make mistakes.
- That's the right attitude, especially in such a gloomy time when there's only negativity all around. - The samurai glanced at the kunoichi. - Zumi, I see that you've become even more beautiful than before. It's a pity that your skills are unlikely to be useful against this enemy.
...

Added 22 minutes ago

Does the decoration slot in the house hold various pictures, clocks, lamps, etc., or furniture? Because furniture seems to provide a slot.

Only furniture provides a slot.

And how often can you change items in inventory slots? For example, in the "other" slot.

Before the start of battles, entering "unsafe" locations, etc.

Is it a thrown weapon, or how does it work?

It attacks with magic.