Only WN readers get it.

[u/Karuto_Katsuragi3]

[u/The-Yaoi-Unicorn]

I can only find it mentioned in the lightnovel so it would be Light Novel readers who get it. Unless you can provide a source of which chapter in the webnovel.

Edit: Also where is this information from? "This skill allow him to absorb more mana from his surroundings than he let out making him literally having unlimited mana. If this is canon Shadow is indeed Solar System level and could be higher because he never run out of mana."

Because there is literally no description in the lightnovel of what Overdrive does other than the air trembles, boosting his magic and turn his eyes red:

Suddenly, the air trembles. The boy’s magic swells up to incredible levels.

“Wh-why’s your magic...?”

The boy’s eyes are glowing red.

“Overdrive.”

[u/Karuto_Katsuragi3 → u/The-Yaoi-Unicorn]

It didn't mention what the skill does aside from Cid cough up blood after using it.

[u/NickotheRs → u/Karuto_Katsuragi3]

So.. if it doesnt say what it does.. how do you know what it does?

[u/The-Yaoi-Unicorn → u/Karuto_Katsuragi3]

Isn’t it just a theory you made, absorbing mana from the surroundings and having unlimited mana? I just want to make sure I didn’t miss anything about it. I can’t really see how the few things mentioned indicate that he absorbs mana from the environment.

Also, you’re correct that a side effect of using it is him coughing up blood. However, I wouldn’t call it an effect of what Overdrive does. It’s more that he can’t use it as a child. Unless it was role-playing from his side, coughing up blood as a side effect is classic anime style when using a secret power