The Crystal Sentry spawns, already affiliated with your team, from your Vain Crystal at the 4 minute mark and walks out to the Crystal Mine in your side of the Jungle.
The Crystal Sentry is a protector in your Jungle, automatically targeting and attacking any enemy hero on sight. It travels a significant distance and deals significantly more damage to help you defend against enemy invasions.
The Crystal Sentry may even fight in lane with you if the enemy or the Kraken is pushing while it respawns, but it deals 30% damage to Kraken.
The bounty for killing a Crystal Sentry fairly significant at 125 gold, and you can gain some experience, but it is a tough fight. The Crystal Sentry only has 3 lives, after which it will no longer respawn after dying.