The rune berserker shield is a reward that can be purchased with 300 tokens from the Fist of Guthixminigame. It can also be bought (in its uncharged state) on the Grand Exchange and recharged for 30 tokens. If you buy it from the Grand Exchange you can still charge it for 30 tokens; just 'use' it on Reggie the rewards trader. It requires 55 defence to wield. When charged, it gives higher armour bonuses than the Rune kiteshield and Rune platebody.
Like Barrows equipment its bonuses will only decrease after the shield has completely degraded to 0% (after only 5 hours of solid combat), but the Barrows equipment takes 15 hours to degrade completely. When completely degraded, or uncharged, it will have no defensive capability and become tradeable. It costs 30 Fist of Guthix tokens to recharge the Rune Berserker 0 shield.
Although this is the joint 2nd best melee shield for F2P (tied with gud raider shield; corrupt dragon sq shield has the best defensive stats for a melee shield for F2P), it is rarely used because of the lack of players playing Fist of Guthix.