To get the Song of the Deep relic in the game Fisch on Roblox, the only way is to complete a quest given by Captain Ahab. Captain Ahab can be found on Moosewood island near the dock. He will ask you to bring him a special and very rare fish called Moby. You can catch the Moby fish during the Whale Migration event, which occurs randomly and has about a 5% chance of Moby appearing. This event can be increased in likelihood by using Tempest and Smokescreen totems, especially during rainy weather. The Moby fish prefers squid as bait, spawns better in the summer season, and requires a high-tier rod with a large max KG stat to catch it. Once you catch Moby, return to Captain Ahab and trade the fish to receive the Song of the Deep relic and the Moby bobber. Using the Song of the Deep relic on a Keeper's Altar grants the Blessed Song enchant, which boosts progress speed by 50% on your rods. However, enchanting success may vary, and improper use can remove current enchantments on your rod. Summary:

• Find Captain Ahab on Moosewood near the dock.
• Get Moby fish during Whale Migration event (rare, 5% chance).
• Use Tempest/Smokescreen totems to increase event spawn chances.
• Catch Moby with high-tier rod and squid bait.
• Trade Moby with Captain Ahab to get the Song of the Deep relic.
• Use the relic to get Blessed Song enchant (progress speed boost).

This method provides the main and only way to get the relic currently in Fisch.