Dark Legend

Do you know how to encounter other legendaries?

No. Not yet, but if you look it up, you might learn it. I'm not going to though, because I want it to be a surprise.

I looked this up though:

Here is all the legendary pokemon in Brilliant Diamond:

Both games:

Azelf – main story; Lake Valor
Mespirit – main story; Lake Verity
Uxie – main story; Lake Acuity
Giratina – main story; Stark Mountain
Heatran – main story; Turnback Cave
Groudon – Ramanas Park
Kyogre – Ramanas Park
Rayquaza – Ramanas Park
Regice – Ramanas Park
Regirock – Ramanas Park
Registeel – Ramanas Park
Mewtwo – Ramanas Park
Mew – interact with NPC in Floaroma Town (need Let’s Go Eevee! or Let’s Go Pikachu! save files)
Jirachi – interact with NPC in Floaroma Town (need Pokémon Sword & Shield save files)
Manaphy – Mystery Gift (expires February 2022)
Phione – by breeding; place Manaphy and Ditto in the nursery
Shaymin (Land) – Seabreak Path Event; Route 224 / Flower Paradise
Shaymin (Sky) – Seabreak Path Event; Route 224 / Flower Paradise
Darkrai – Event; Canalave City / Newmoon Island
Arceus – Event
Celebi – TBC
Creselia – Fullmoon Island (*roaming Pokémon)
Deoxys – TBC
Regigigas – TBC
Legendary Pokémon exclusive to Brilliant Diamond
Dialga – main story; Spear Pillar
Entei – Ramanas Park
Raikou – Ramanas Park
Suicune – Ramanas Park
Ho-Oh – Ramanas Park

Congratulations

Thank you! I hope to explore the great underground's secret areas, as I didn't get a chance to during my playthrough.