Here is the full list of all 46 Sker Ritual achievements.
Defeat Isaac
Escort all Lambs in under 5 minutes
Have all the perks activated at once
Get a Miracle to Legendary
Defeat Abraham
Visit every location in the Cursed Lands of Lavernock
Survive Round 20
Escort the Lamb to slaughter in under 2 minutes
Survive Round 35
Survive Round 50
Find all the dolls in Lavernock
Rank a gun to max level
Visit every location in The Ashes of Sker Hotel
Find all the dolls in Sker Hotel
Survive a Dark Round without receiving damage
Visit every location in the Sewers of the Dead
Defeat The Stranger
Find all the dolls in Sewers of the Dead
Escort the Quiet One in under 2 minutes
Complete the radio tower objective without receiving damage
Visit every location in the Deadly Lover's Fortress
Defeat Elisabeth and Thomas
Find all the dolls in Deadly Lover's Fortress
Complete "Gates of the Damned Part I" without receiving damage
Complete the "Weird Science" objective in under 4 minutes
Find all lore notes in Cursed Lands of Lavernock
Find all lore notes in The Ashes of Sker Hotel
Find all lore notes in Sewers of the Dead
Find all lore notes in Deadly Lover's Fortress
Complete Cursed Lands of Lavernock under 30 minutes
Complete The Ashes of Sker Hotel under 25 minutes
Complete Sewers of the Dead under 35 minutes
Complete Deadly Lover's Fortress under 35 minutes
Complete the hidden objective in Cursed Lands of Lavernock
Complete the hidden objective in The Ashes of Sker Hotel
Complete the hidden objective in Sewers of the Dead
Complete the hidden objective in Deadly Lover's Fortress
Complete Cursed Lands of Lavernock on Hard Difficulty
Complete The Ashes of Sker Hotel on Hard Difficulty
Complete Sewers of the Dead on Hard Difficulty
Complete Deadly Lover's Fortress on Hard Difficulty
Complete Cursed Lands of Lavernock on Nightmare Difficulty
Complete The Ashes of Sker Hotel on Nightmare Difficulty
Complete Sewers of the Dead on Nightmare Difficulty
Complete Deadly Lover's Fortress on Nightmare Difficulty
Complete all levels including the Hidden objectives on Nightmare