MTGNexus

Rishadan Port

WOTC Ruling 03/16/18 "Once you announce that you're casting a spell or activating an ability, no player may take other actions until the spell or ability has been paid for. Notably, players can't try to tap your lands to stop you from paying."

So, like, is there a particular phase where you can tap someone's land like a Gaea's Cradle to prevent the opponent from tapping it for land in response? At the end of the untap phase at instant speed, because technically the mana pool would empty before first main phase? If so, would the opponent would only have the option of casting instants before first main phase with the generated mana?

You basically have it right with your second paragraph (though the timing is slightly off). Let's take the example of Gaea's Cradle:

Say you are playing against Elves and they have a Cradle and 4 Elves on the field. You want to make sure they can't use that mana to do much on their turn. So, in their Upkeep, right after they untap, you will get priority to tap their land. This does allow them to respond and tap the Cradle for 4 mana but that mana is only useful while still in the Upkeep. They could only use it to activate abilities or cast Instants or spells with Flash.

When they get to their Main Phase, that mana is unavailable to them.

The ruling you quoted is actually telling the Port player what they can/can't do. That is, you can't wait for them to get to their Main Phase, start casting a spell, and then tap the Cradle before they pay for the spell so they don't have the mana. You don't have priority in the middle of a spell being cast (you will get priority after it is cast, and paid for, but before it resolves).

Ok, the ruling makes sense. So the Rishadan Port can tap an opponents land in a phase before a main phase. Similarly to when the port player has priority at the end of the opponents Upkeep phase, the port player can tap a land before the end of the combat phase to prevent the opponent from using that mana in the second main phase because the mana will empty therefore only allowing instants, flash, and abilites available for mana use before it empties.

Sure. Though, Upkeep is a Step, not a Phase (it is part of the Beginning Phase). And yes, you can tap the land during Combat during any of the 3-5 steps (depending on whether they declared attackers) to prevent them access to that mana in the second main phase if you want to wait that long. That does give them access to it in the Precombat Main Phase though so tapping it in Upkeep is usually correct.

The advice that using Port in their upkeep is sound. My response wondering about the combat phase to affect second main, was because say somehow I could untap my port and pay for it again it could work that way.