Herald of Leshrac — ruling 1

The cumulative upkeep trigger doesn't target what you gain control of, and you don't choose lands until the ability resolves. Your opponents can not tap lands in response to the choice, but they could respond to the ability by tapping any number of lands before you've made any choices.

Herald of Leshrac — ruling 2

When Herald of Leshrac's cumulative upkeep ability resolves, if its age counters outnumber the number of lands on the battlefield that you don't control, you can't pay its cumulative upkeep cost and must sacrifice it.

Herald of Leshrac — ruling 3

You must choose a different land you don't control for each age counter on Herald of Leshrac. Otherwise, you'd try to gain control of a land you *do* control midway through paying the cost and need to back up.

Herald of Leshrac — ruling 4

Herald of Leshrac's leaves-the-battlefield ability affects all lands you control but don't own, not just the ones you gained control of with Herald of Leshrac. For example, if you had gained control of an opponent's land with Annex, its owner will regain control of that land. Annex will remain attached to it, but its effect will be overridden.

Herald of Leshrac : règles, fonctionnement et interactions (MTG)

Herald of Leshrac is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

Flying Cumulative upkeep—Gain control of a land you don't control. (At the beginning of your upkeep, put an age counter on this permanent, then sacrifice it unless you pay its upkeep cost for each age counter on it.) This creature gets +1/+1 for each land you control but don't own. When this creature leaves the battlefield, each player gains control of each land they own that you control.

How it works : Herald of Leshrac

Original English text — Wizards of the Coast / Scryfall

#1April 15, 2013· Wizards of the Coast
The cumulative upkeep trigger doesn't target what you gain control of, and you don't choose lands until the ability resolves. Your opponents can not tap lands in response to the choice, but they could respond to the ability by tapping any number of lands before you've made any choices.
No other card is named in this ruling.
#2July 15, 2006· Wizards of the Coast
When Herald of Leshrac's cumulative upkeep ability resolves, if its age counters outnumber the number of lands on the battlefield that you don't control, you can't pay its cumulative upkeep cost and must sacrifice it.
No other card is named in this ruling.
#3July 15, 2006· Wizards of the Coast
You must choose a different land you don't control for each age counter on Herald of Leshrac. Otherwise, you'd try to gain control of a land you *do* control midway through paying the cost and need to back up.
No other card is named in this ruling.
#4July 15, 2006· Wizards of the Coast
Herald of Leshrac's leaves-the-battlefield ability affects all lands you control but don't own, not just the ones you gained control of with Herald of Leshrac. For example, if you had gained control of an opponent's land with Annex, its owner will regain control of that land. Annex will remain attached to it, but its effect will be overridden.
No other card is named in this ruling.