History of Tablet of Ruin

The Tablet of Ruin is the seasonal artifact of Episode: Heresy. Destinypedia's lore entry says it was recovered from Savathûn's Throne World when Alak-Hul was slain.

Acquisition

It is unlocked automatically after the Episode: Heresy prologue missions (Espial and Recce, per Deltia's/Sportskeeda-style guides found in search results; the Destiny 2 Wiki says after the introductory mission).

Artifact

The Destiny 2 Wiki lists 35 perks in five tiers, of which up to 12 can be active, covering weapon-specific effects, Arc, Void and Strand synergies and ability-energy utility.

Altar of Reflection

Altar of Reflection: Insight is described in game as revisiting "the memory linked to the Tablet of Ruin". Note: the item is a Heresy artifact, not a Destiny 2: The Witch Queen quest object, although the altar activity itself dates from that expansion's Throne World.