How to get the Legendary Mace in Baldur's gate 3, The Blood of Lathander.

Go to the Monastery, on the second level there is a weapon puzzle. The weapons are all around the building. Some regular weapons work also.

Once the weapons are in the right spots, you can collect the Dawnmasters Crest. Take the crest into the Creche. Follow to the secret statues that you can rotate. You may need to grease the left statue to make it turnable.

Proceed down the opened path to the force walls and traps puzzle. Just destroy each of the Crystals powering the doors. Once through, place the Dawnmasters Crest in the slot to retrieve the mace