Frostwalk Bastion
Snow Land
: Add 
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: Until end of turn, this land becomes a 2/3 Construct artifact creature. It's still a land. ( can be paid with one mana from a snow source.)
Whenever this land deals combat damage to a creature, tap that creature and it doesn't untap during its controller's next untap step.
Jedd Chevrier
Modern Horizons (MH1)
• #240Rulings
A creature dealt combat damage by Frostwalk Bastion won't untap during its controller's next untap step, even if it was already tapped and even if Frostwalk Bastion leaves the battlefield before then.
(wotc • 2019-06-14)
If you turn Frostwalk Bastion into a creature but haven't controlled it continuously since your most recent turn began, you won't be able to activate its mana ability or attack with it.
(wotc • 2019-06-14)
Rulings
A creature dealt combat damage by Frostwalk Bastion won't untap during its controller's next untap step, even if it was already tapped and even if Frostwalk Bastion leaves the battlefield before then.
(wotc • 2019-06-14)
If you turn Frostwalk Bastion into a creature but haven't controlled it continuously since your most recent turn began, you won't be able to activate its mana ability or attack with it.
(wotc • 2019-06-14)