Increasing Stranglegasp drop change

Hi,

How is Stranglegasp drop chance affected by, em, things ?

Is it just some 0,0X% chance to drop when completing the map, in which case there is nothing to do ? Is it affected by iiquantity ? iirarity, for some reason ?
Is it boss-dependent, so increasing the number of bosses spawning could help ? Monster density ?

Or maybe I missed another way to increase drop chance ?

Any help or clarification is appreciated

It's a rare drop from Blight Ravaged maps. That's all we know really.