20 Sept 2016
If you no longer control an artifact with the highest mana value as Padeem's triggered ability resolves, you don't draw a card.
20 Sept 2016
The artifact you control has to have the highest mana value only among artifacts on the battlefield, not among all permanents on the battlefield.
20 Sept 2016
If you don't control an artifact with the highest mana value as your upkeep begins, Padeem's second ability won't trigger. You can't cast an instant to destroy an opponent's artifact during your turn before your upkeep begins.
20 Sept 2016
If an artifact has {X} in its mana cost, X is considered to be 0.