The Grappler Bow in Fortnite is essentially a combination of the Boom Bow and the Grappler. The Grappler, introduced in September of 2018, could be shot at items (within a certain distance), and the player would be yanked towards it (fall damage applied).
The Boom Bow, introduced in March 2019, was a crossbow weapon that fired explosive shots at players and structures. The Grappler Bow worked almost the same way - it could shoot players or grapple towards things.
The Grappler Bow was introduced for Chapter 2, Season 6, and was tied with Lara Croft. Croft was an NPC that was part of the battle pass. She lived at Stealthy Stronghold and, more importantly, sold the Grappler Bow for 500 gold bars.
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The Grappler Bow: Available in Fortnite Chapter 2 Season 7?
Fortnite has often made their crossover NPCs and added them to the game. This season, Batman, Superman, and Rick Sanchez all play vital roles in the storyline. Last season, it was Raven and Lara Croft. Croft's role was essentially to sell an Exotic weapon - the Grappler Bow.

In a new season, all new NPCs and all-new weapons are being sold by them. Doctor Slone holds the Pulse Rifle. Guggimon sells the Night Hawk, and others have Exotic weapons as well. With all the new NPCs moving onto the map, the old NPCs are no longer a part of the game, including Lara Croft.
Croft left when the season rolled over into Season 7 and with her the Grappling Bow. Neither Croft nor the Grappling Bow is available in-game for Fortnite Chapter 2 Season 7. It is currently in the vault and may eventually be added back to the active weapons roster. Fortnite usually doesn't vault things for good so that it may be back someday, but not during Season 7.