What the source confirms
The official experience thumbnail visibly names Spike Ball and labels it Legendary. That makes the identity and rarity safe to publish.
Role hypothesis to test
A confirmed Legendary weapon worth testing against dense wave pressure.
What remains unknown
Legendary rarity alone does not prove a universal S-tier ranking.
Run a controlled comparison: keep the rest of your loadout stable, change this slot, and record what ends the run. That result is more useful than a copied tier label with no patch or source.