Serpentine 2h crossbow was removed from RuneScape after an update, but still exists in-game for those who had obtained it.
The serpentine 2h crossbow was obtainable as a very rare prize on Treasure Hunter, or purchased for 1,250 Rare item tokens from the Rare token store. It is a tiered weapon, with level requirements of 1, 20, 40, 60 and 70 Ranged. At level 70 it is among the most powerful non-degrading 2-handed crossbows in existence (on par with the blisterwood stake-thrower crossbow). It shares the same stats with Karil's crossbow, but its ammo is not limited to bolt racks.
During the weekend of its introduction (24 to 27 January 2014 inclusive) there was an increased chance to obtain it. When the player wins it, they receive a level 1 crossbow, which can be changed to level 20, 40, 60 and 70. Anyone can change the crossbow to any level, but it can only be wielded with the required ranged level. If destroyed, it can be reclaimed from Diango for a fee of 10,000–100,000 coins (depending on your level).