Boss
How to beat Death
Do not farm the first fight
The Altar of Bones in the Catacombs is a story fight. Listed health is 20,000. It does not drop a card. The fight that matters for traversal is in the Notre-Dame Choir, after the three organ melodies. Listed health there is 8,444 and experience is 1,000.
The Choir fight drops The Shadow, the Soul Harvest spell, and the double jump. Rock Paper Shotgun places that unlock around 60 percent. A search for a 'death clock' mechanic is not supported. This page does not describe a clock.
The one-hit tell
When his fingers glow green, the next hit is a kill. After he vanishes, a drawn line shows where he will reappear. Those two tells are the pattern notes the sources actually print. Frame data beyond that is not here.
What I ran into
I spent a long time in the catacombs wondering why double jump had not appeared. The health bar was enormous and the reward was nothing. The Choir is the fight I should have been reading about.
Questions
Where do I fight Death for the double jump?
The Choir in Notre-Dame, after the three organ melodies. Not the Altar of Bones.
What card does he drop?
The Shadow, from the Choir fight only, along with Soul Harvest and double jump.