When does this power trigger exactly : at the beginning of its activaion ?
When Atlas is activated, his owner must decide wether or not to discard an additional card (of any type).
If they choose not to, each opposing player draws 1 card.
I think it should be written "At the beginning of its activation".
"When Atlas is activated" means exactly the same as "At the beginning of its activation..." You can expound it to "At the moment/ time when Atlas is activated..." but that is already too long. "When Atlas is activated..." is already a clear way of saying it.
If the intent was to discard a card at anytime or at the end, the wording would be:
Anytime : "At anytime during Atlas' activation..."
End : "At the end of Atlas' Activation..."