Why does takshak need arjun?

Ok so as far as i understand takshak was the one who ordered astika to marry and have a son with pranalini. The fact that astika is his son-in-law bothers me as why would he ruin his own daughter, mohini's marital life by ordering her husband to knock-up another woman? Why does takshak want astika to find his son and the naagmani. Why was the desire for astika to have a son with a human woman and for the son to be found so big for takshak? Can someone explain takshak's motives to me please? 

There was a prophecy in naaglok that a son born to Astika from a woman would be able to bear and utilise or manipulate the powers of the naagmani. And takshak with intention of becoming the ruler of all the dimensions wants to use Astika's human child as a weapon against all the divine forces defending mankind. So yes for takshak the desire for power was greater than his daughter's marital life.

Originally posted by: Rein123

There was a prophecy in naaglok that a son born to Astika from a woman would be able to bear and utilise or manipulate the powers of the naagmani. And takshak with intention of becoming the ruler of all the dimensions wants to use Astika's human child as a weapon against all the divine forces defending mankind. So yes for takshak the desire for power was greater than his daughter's marital life.

Originally posted by: sammy17

Oh okay so that makes so much sense now. So arjun will be able to use the powers of the naagmani more efficiently than anyone else would so that's why he is needed. That makes things more interesting. I do have one more question though. is arjun actually half human- half naag or are his naag abilities strictly owing to the fact he possesses the naagmani?

Arjun's father is astika a nag and his mother pranali a human.
His possess nag power becoz lineage.