Obelisk the Tormentor's Effect
* Obelisk's Effect: It has the effect, "This card cannot be destroyed by battle." This means that it cannot be destroyed during a battle phase.
* Obelisk's Attack/Defense: Its attack and defense are based on the number of monsters on the field. It's a powerful monster!
Your Questions
* Cloudian - Eye of Typhoon: Cloudian - Eye of Typhoon cannot be destroyed by battle, but Obelisk's effect does not destroy monsters in battle. Obelisk's effect states that it cannot be destroyed by battle. It does not prevent other cards from destroying monsters. So, while Cloudian-Eye of Typhoon is safe from battle destruction, it can still be destroyed by other card effects.
* Obnoxious Celtic Guard: Obelisk the Tormentor cannot destroy Obnoxious Celtic Guard in battle, since Obelisk's effect only prevents itself from being destroyed by battle. However, Obelisk the Tormentor can destroy other monsters during battle and can even destroy Obnoxious Celtic Guard by using its own card effect, "This card inflicts piercing damage to your opponent's Life Points." This means that even though Obnoxious Celtic Guard can't be destroyed by battle, it can still be destroyed by Obelisk's piercing damage.
* Copying Obelisk's Attack and Defense: While many cards in Yugioh allow you to copy a monster's attack and defense, Obelisk the Tormentor specifically states that its attack and defense are calculated based on the number of monsters on the field. This means you cannot copy its attack and defense with cards that normally copy these stats.
In short, Obelisk's effect is a powerful defense for itself, but it doesn't affect other monsters' destruction. It can still deal piercing damage, which allows it to destroy even monsters that cannot be destroyed by battle. While its attack and defense are based on the number of monsters on the field, it cannot be copied by effects that typically copy those stats.