Boss
How to beat Isaac
Two fights, one card
The Castle Drawbridge in the Main Hall is a phantom. Listed health is 1,500 and experience is 750. You do not absorb a card. The real Isaac is at the Heart of the Clock, at the top of the Clock Tower. Listed health is 2,500 and experience is 1,000. That fight drops The Zealot, and the spell is Chronomantic Dive.
Konami's character page calls him a devil forgemaster with a grudge against Trevor from twenty years earlier. Liam O'Brien voices him. He is the last fight in the tower before Dracula.
Getting there
A turn-by-turn from the Main Hall door to the Heart of the Clock is not in the sources. The named landings are Foundry Apex, Mechanic's Ascent, and Eastern Summit. Use those on the Clock Tower map, then climb. I am not filling the shaft with invented platforms.
What I ran into
I beat the drawbridge phantom and waited for a card that never came. That is the whole reason this page leads with 'two fights' instead of a combo list.
Questions
Where is Isaac?
The card fight is at the Heart of the Clock. An earlier phantom is on the Castle Drawbridge.
What does Isaac drop?
Only the Heart of the Clock fight drops The Zealot and Chronomantic Dive.