The seercull is a Fremennik bow that requires a Ranged level of 50 to wield, and fires arrows up to Amethyst. Compared to a magic shortbow, it has a slower rate of fire and a different special attack which is more effective against mages. In addition, its normal and rapid fire attacks have one more range than that of a shortbow's, and its long range attack is equivalent to that of a composite bow's. It is a comparable weapon to the Magic comp bow.
The name can be attributed to the combination of the word seer (another word for "mage" or "wizard") and the word cull (another word for "kill"). In other words, the mage-killer. This is a fitting name because of the effect of its special attack and its examine text.
A player wielding the seercull.
A player using the bow's special attack, Soulshot
The Seercull has a special attack, Soulshot, which consumes 100% of the player's special attack energy. It is guaranteed to hit and lowers the opponent's Magic level by the same as the amount of damage dealt. However, damage inflicted by Soulshot is unaffected by Ranged prayers (such as Eagle Eye and Rigour) and the slayer helmet (i), and the special attack won't drain an opponent that's already been drained, so only your first special attack will have any effect on its level.
Due to the nature of its special attack, the Seercull is not commonly used in combat outside of PvP. Rarely would a special attack that lowers the magic level come in use while training or fighting bosses, especially not this special attack as, unlike that of the Bandos Godsword or the Dragon Warhammer, this special attack will only drain it's opponent's levels once.
In PvP combat, the special attack gains use for its ability to lower the Magic level of a mage to prevent them from casting spells. However, this effect is likely to be mitigated by use of super restore potions, which are commonly used in lieu of prayer potions.
For an exhaustive list of all known sources for this item, see here