> Njáll hefir því valdið þá

"Nj. is responsible for that then."

'valdið' = ON 'valdit', past participle neuter of 'valda e-u' "to
cause, be responsible for something". (Rather than 'valið' = 'valit',
past participle neuter of 'velja' "to choose".)

'því' "that" (dative singular neuter pronoun) is the object of
'valdið', the thing that he caused.