Sword of Feast and Famine
Artifact — Equipment
Equipped creature gets +2/+2 and has protection from black and from green.
Whenever equipped creature deals combat damage to a player, that player discards a card and you untap all lands you control.
Equip 
Efrem Palacios
Grand Prix Promos (PGPX)
• #2016bRulings
You'll untap all lands you control even if the player can't discard a card.
(wotc • 2020-08-07)
You can tap your lands for mana while the triggered ability is on the stack. However, you'll lose any mana you produce this way if you don't spend it before the end of the current combat damage step.
(wotc • 2020-08-07)
Rulings
You'll untap all lands you control even if the player can't discard a card.
(wotc • 2020-08-07)
You can tap your lands for mana while the triggered ability is on the stack. However, you'll lose any mana you produce this way if you don't spend it before the end of the current combat damage step.
(wotc • 2020-08-07)