Someone plz explain the effect off Ally of catastor effect I dont understand it.

It activates at the beginning of the damage step, it does not work on face down monsters. It can also be negated I believe during the BP by cards like Masquerade. But it pretty much activates all the time.Flipping Catastor will also cancel out his effect too if you attack him .

(I still think i'm wrong about this myself. I'm still quite noobish but I tried )

Ally of Justice Catastor is a weird card. There are ways around it though. Like Don said, facedown monsters are unaffected due to the fact that they are flipped after the start of the damage step. If it's battling a monster that is treated as two attributes (dark and wind of Dark Simorgh for example) then Catastor would still destroy it with it's effect. It's also a mandatory effect, meaning that you don't have the option to not destroy a non-dark monster by it's effect, it just happens. There's ways around it though. Having Fiendish Chain on Catastor negates it's effect and during the Damage step you can use Mist Valley Thunder Lord's effect to bring chain back to your hand and giving Thunder Lord +500. This'll bypass the start of the damage step allowing Thunder Lord to destroy Catastor in battle. Another popular way is using Neo-Spacian Grand Mole. Catastor's effect would chain 1 and Mole could chain 2 to that, thus getting bouncing Catastor back to the extra deck. Catastor is powerful, but not invincible.

Fluff Fluff wrote:

Having Fiendish Chain on Catastor negates it's effect and during the Damage step you can use Mist Valley Thunder Lord's effect to bring chain back to your hand and giving Thunder Lord +500. This'll bypass the start of the damage step allowing Thunder Lord to destroy Catastor in battle. Another popular way is using Neo-Spacian Grand Mole. Catastor's effect would chain 1 and Mole could chain 2 to that, thus getting bouncing Catastor back to the extra deck. Catastor is powerful, but not invincible.

Geez your really obsessed with mist valley huh? Anyway i thought mole dies from Catastors effect ?

Lol it's growing on me :s But since Catastor is a mandatory effect it'll have to happen. Say you have Mole and your opponent has Catastor. Catastor attacks and your opponent attempts to use it's effect to destroy Mole. Since they both have the same activation time (At the start of the damage step) you can chain Mole's effect to return Catastor to extra as chain link 2. It'll resolve first and thus Catastors effect will resolve without anything happening since Mole and Catastor would be back in hand/extra deck.

Ah okay thank you guys I get it

Simply because chains resolve backwards.. haha