Nemesis of Mortals — ruling 1

Neither the cost-reducing ability of Nemesis of Mortals nor the condition of its monstrosity ability can reduce the colored mana requirement of those costs.

Nemesis of Mortals — ruling 2

Once you declare you're casting Nemesis of Mortals or activating its ability, it's too late for players to try to change the number of creature cards in your graveyard to affect the cost of doing so.

Nemesis of Mortals — ruling 3

Once a creature becomes monstrous, it can't become monstrous again. If the creature is already monstrous when the monstrosity ability resolves, nothing happens.

Nemesis of Mortals — ruling 4

Monstrous isn't an ability that a creature has. It's just something true about that creature. If the creature stops being a creature or loses its abilities, it will continue to be monstrous.

Nemesis of Mortals — ruling 5

An ability that triggers when a creature becomes monstrous won't trigger if that creature isn't on the battlefield when its monstrosity ability resolves.

Nemesis of Mortals : règles, fonctionnement et interactions (MTG)

Nemesis of Mortals 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

This spell costs less to cast for each creature card in your graveyard. : Monstrosity 5. This ability costs less to activate for each creature card in your graveyard. (If this creature isn't monstrous, put five +1/+1 counters on it and it becomes monstrous.)

How it works : Nemesis of Mortals

Original English text — Wizards of the Coast / Scryfall

#1September 15, 2013· Wizards of the Coast
Neither the cost-reducing ability of Nemesis of Mortals nor the condition of its monstrosity ability can reduce the colored mana requirement of those costs.
No other card is named in this ruling.
#2September 15, 2013· Wizards of the Coast
Once you declare you're casting Nemesis of Mortals or activating its ability, it's too late for players to try to change the number of creature cards in your graveyard to affect the cost of doing so.
No other card is named in this ruling.
#3September 15, 2013· Wizards of the Coast
Once a creature becomes monstrous, it can't become monstrous again. If the creature is already monstrous when the monstrosity ability resolves, nothing happens.
No other card is named in this ruling.
#4September 15, 2013· Wizards of the Coast
Monstrous isn't an ability that a creature has. It's just something true about that creature. If the creature stops being a creature or loses its abilities, it will continue to be monstrous.
No other card is named in this ruling.
#5September 15, 2013· Wizards of the Coast
An ability that triggers when a creature becomes monstrous won't trigger if that creature isn't on the battlefield when its monstrosity ability resolves.
No other card is named in this ruling.