Learn Swedish: Vad som än händer borde ni prata om det osagda. - Whatever happens, you should talk about the unsaid.

In Swedish, you can drop the noun completely when referring to an abstract concept. By putting the neuter definite article det in front of an adjective, it transforms into a noun phrase meaning "that which is [adjective]". This is why det osagda translates to "the unsaid" or "what has not been said". You will frequently see this pattern with phrases like det bästa (the best thing) and det viktiga (the important thing).

Why does osagda end in -a if det refers to a singular, abstract concept?

When you turn an adjective into a noun using det (a substantivized adjective), the adjective must still take its definite form, which ends in -a. Even though it represents a singular abstract concept, it follows the exact same ending rule as a regular definite adjective describing a neuter noun, just like in det stora huset (the big house).

Why is som needed in Vad som än händer? Can I just say Vad än händer?

You need som because vad is acting as the subject of the clause "whatever happens". In Swedish, whenever a question word like vad (what) or vem (who) is the subject of a relative clause, you are grammatically required to insert som right after it. You can think of it literally as "What that ever happens".