Relics are basically those hearts that are on the left of the character menu. The achievement requires us to collect 10.
I know of the existence of 20 relics, it is possible that there are more than that. We won one of them at the end of the tutorial and I'm going to put a video from the "Shummell Gaming" channel that contains 16 others. I recommend that you check the name of the map if you have it open in your campaign and give preference to them. If any are missing later, start an adventure in the world that is most convenient to get more:
0:00 Runed Heart (Labirint)
0:59 Reprocessed Heart (Labirint)
2:00 Unsullied Heart (Losomn - Fae Imposter Storyline)
4:08 Quilted Heart (Losomn - Morrow Parish)
5:40 Pulsing Heart (Yaesha - Endaira's End)
8:24 Blooming Heart (Yaesha - The Expanding Glade)
9:59 Lifeless Heart (Yaesha - Ravenger Storyline)
14:07 Ripened Heart (Yaesha - The Widow's Court)
16:50 Salvaged Heart (N'Erud - Ascension Spire)
17:56 Crystal Heart (N'Erud - The Dark Conduit)
18:51 Siphon Heart (N'Erud - Forgotten Prison - Tal'Ratha Storyline)
20:14 Shielded Relic (N'Erud - Tower of the Unseen)
22:56 Void Heart (N'Erud - Alepsis-Taura Storyline)
23:45 Decayed Heart (Corrupted Earth - Corrupted Harbor)
25:16 Resonating Heart (Corrupted Earth - Ashen Wasteland)
26:14 Tormented & Tranquil Heart (Nightweaver Story in Losomn + Timeless Horizon Story in N'Erud in the same playthrough)
PS: I had to collect 12 to drop the achievement.