Thunderblade Charge — ruling 1

When the ability triggers, first you choose whether to pay {2}{R}{R}{R}. If you paid, then you choose whether to cast the spell.

Thunderblade Charge — ruling 2

Thunderblade Charge triggers only once per combat damage step per player no matter how many of your creatures deal combat damage to that player, how much damage is dealt, or who that player is. (It might be you if the combat damage is redirected.)

Thunderblade Charge : règles, fonctionnement et interactions (MTG)

Thunderblade Charge 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

Thunderblade Charge deals 3 damage to any target. Whenever one or more creatures you control deal combat damage to a player, if this card is in your graveyard, you may pay . If you do, you may cast it without paying its mana cost.

How it works : Thunderblade Charge

Original English text — Wizards of the Coast / Scryfall

#1May 1, 2007· Wizards of the Coast
When the ability triggers, first you choose whether to pay . If you paid, then you choose whether to cast the spell.
No other card is named in this ruling.
#2May 1, 2007· Wizards of the Coast
Thunderblade Charge triggers only once per combat damage step per player no matter how many of your creatures deal combat damage to that player, how much damage is dealt, or who that player is. (It might be you if the combat damage is redirected.)
No other card is named in this ruling.