What happened to Castrol India stock opened 50% down

[Tintin]

Answered in detail here:

Castrol India Ltd goes ex-bonus in the ratio 1:1 on 21 Dec 2017. How does this affect my equity holdings and F&O positions?

CASTROLIND goes ex-bonus in the ratio 1:1 on 21 Dec 2017. How does this affect my equity holdings and F&O positions?