Puzzle 1 - Trial of Flame
The first puzzle in Pillars of Eternity 2 Deadfire can be found in Engwithan Digsite (M8), in Arena sub-level (M9) - this puzzle is called Trial of Flame. The map above shows the exact location of the puzzle.
How to solve the Trial of Flame puzzle? Start with setting flames on the two sides of the blocked entrance - use either a torch or a flint and tinder (torches can be found in the same location as the puzzle).
Then, you have to stand on the right pressure plates - plates 1 and 3 by going from the upper row downwards (the one on the left), and the plate 1 from the bottom row. The solution can be found in the picture above.
This opens the locked passage. Inside, you can find a chest with a Gladiator Sword and a Jade.