Learn German: Die Fusion zweier Unternehmen ist kompliziert, was mich nicht überrascht. - The merger of two companies is complicated, which does not surprise me.

Zweier is the genitive form of zwei. Because there is no article before the noun (like der or dieser) to show the genitive case, the number itself takes the genitive ending -er to express 'of two companies'.

Since zweier means 'of two', why doesn't Unternehmen have a plural ending?

The word Unternehmen (company) belongs to a group of German nouns ending in -en that do not change their form in the plural. It is das Unternehmen in the singular and die Unternehmen in the plural, so the form remains the same here.

Why is was used here instead of a standard relative pronoun like das?

Here, was refers back to the entire preceding statement (Die Fusion zweier Unternehmen ist kompliziert). When a relative pronoun refers to a whole idea or sentence rather than one specific noun, you must use was.

Why does überrascht go at the very end of the sentence?

The word was introduces a relative clause, which is a type of subordinate clause. In German subordinate clauses, the conjugated verb (überrascht) is always pushed to the very end.