I had a 115 damage Gatling Wand built up to burst the plague lord so I would not have to deal with his final phase. I got him down to 105 hp, and he had two 3/5 Finjas on his side. I thought I had lethal. Nope. It took him down to 1 hp.
Note the Thalnos in play in the second screenshot.
The card seems to be visually modified by Spell Damage, but the number of missiles fired is unaffected.
https://www.reddit.com/r/hearthstone/comments/d8lwys/explain_blizz_the_board_was_empty_and_21_is/
https://us.forums.blizzard.com/en/hearthstone/t/tombs-of-terror-the-gatling-wand-bugged-against-plague-lord/14188
Note the Thalnos in play in the second screenshot.
The card seems to be visually modified by Spell Damage, but the number of missiles fired is unaffected.