Neheb, Dreadhorde Champion

Trample
Whenever Neheb deals combat damage to a player or planeswalker, you may discard any number of cards. If you do, draw that many cards and add that much {R}. Until end of turn, you don't lose this mana as steps and phases end.

Dominaria United Commander

Trample
Whenever Neheb, Dreadhorde Champion deals combat damage to a player or planeswalker, you may discard any number of cards. If you do, draw that many cards and add that much {R}. Until end of turn, you don't lose this mana as steps and phases end.

War of the Spark Promos

War of the Spark

Other effects may cause you to draw more or fewer cards than the number of cards you discarded. In these cases, the amount of mana you add is equal to the number of cards you discarded, not the number of cards you drew.