Talking about his effect, Alphamon is a Security Digimon that will trigger something if he is opened from security stack.
[Security] At the end of the battle, add this card to your hand. Then if you or your opponent have a digimon with [Royal Knight] or [X Antibody] in their type, up to 12 of your opponent's digimon cannot attack players for the turn.
If we are lucky enough to open this card from security stack, we need to see if either we or opponent have any [RoyalKnight] or [X-Antibody] digimon in the battle, only then opponent's digimon cannot attack player or security for the turn, so if Alphamon is opened at the last attack then the effect is useless.
Just in case you forget, currently there is only 2 digimon with [X-Antibody] type, those are Alphamon and [BT5-111] Omega X-Antibody. For [RoyalKnight] type digimon, we have Omnimon, Dukemon, Craniamon, Ulforce, Leopardmon, Kentaurosmon, Crusadermon (Lordknightmon), Jesmon, Dynasmon, Gankoomon, Omegamon X-antibody and Alphamon. (You can search for Royal Knight digimon in our card search with keyword "royal").