What Shadow Clash is
Shadow Clash is a recurring event gamestratwiki lists as a core step in its early-game sequence: 'Complete Shadow Clash whenever available,' alongside redeeming codes and repeating the death-revive loop. No source checked for this page publishes how often it runs or how long it lasts, so check in-game for whether it's currently active rather than assuming a fixed schedule.
What it rewards
Shadow Clash pays out in Weapon Stones and Soul Ring Rerolls, according to gamestratwiki, which describes both currencies as becoming increasingly important as your build develops — Weapon Stones for upgrading swords along the Death Road, Soul Ring Rerolls for improving your equipped rings.
Its overlap with the shadowclash code
A redemption code named shadowclash also exists, but four cross-checked gaming-media sources list it as expired as of this site's Codes page. The code sharing a name with the event doesn't mean the event itself is inactive — check the /codes page for what's currently redeemable.
What we don't know yet
No source checked for this page documents Shadow Clash's trigger schedule, duration, or whether it involves bosses distinct from normal Death Road enemies. Treat any specific timing claim you see elsewhere as unverified until the game or an official source confirms it.