What is the Dark Souls of mathematics?

What is the Dark Souls of mathematics?

p-adic analysis

Calculus

Triple integrals

...

[math subject I last mastered]

obviously as [math subject I mastered before that] is for babbys

p-adic analysis is easy though

Analysis. Each proof is like a boss in DS: once you know the pattern you can beat them easily. In analysis patterns are inequalities or other theorems that you proved before. Finding out which inequality to use for a proof can be a source of great frustration. For higher analysis replace inequality with integral equation or norm, basically you have to invent a special norm and this will solve your problem but without it it's hopeless.

Go on...

well...