If you control a "Blackwing" monster other than "Blackwing - Gale the Whirlwind", you can Special Summon this card (from your hand). Once per turn: You can target 1 face-up monster your opponent controls; that target's ATK and DEF become half its current ATK and DEF.
Special Summoning requires you to already control a Blackwing monster.
The attack halving effect is useful for bringing down powerful monsters and is generic enough to be somewhat useful outside of Blackwing Decks.
Most of the better Blackwing monsters are good at Special Summoning, opening up opportunities for Synchro plays. Some non-Tuner Blackwing monsters can Special Summon themselves when another Blackwing is on the field, allowing you to pair them with Gale the Whirlwind. Blackwing - Sura the Blue Flame and Blackwing - Tornado the Reverse Wind can Special Summon Gale the Whirlwind.
Yeah, I don’t use it’s atk reduction effect nearly as much as I use it to special summon for synchro plays. Most of your opponents monsters end up getting blown up by Raikiri’s effect anyways by that time you get to your battle phase.