The Ultrafun: Halo Forever! (For a day) is an event organized by TrueAchievements.com to raise money for Child's Play. Staffers from the site, split into two teams, will be running through the campaign missions of Halo 3, Halo 3: ODST and Halo: Reach on Legendary mode in 4 player co-op. As if that wasn't challenging enough, the participants are adding Skulls with every donation milestone to make the experience even more gruelling for everyone involved.

At $100, the team will add the IWHBYD Skull. At $150, the Grunt Birthday Skull. At $250, Cowbell. At $500, Catch. At $1,000, Fog. At $2,500, Tough Luck. At $4,000, Famine. At $6,500, Thunderstorm. At $10,000, Tilt. At $14,000, Black Eye. And at $20,000, the Mythic skull in combination with all the others will make an already difficult challenge next to impossible.

The players will start playing at 1pm BST (8am EST) on July 2 and won't be stopping for 24 hours. If donations pass $50,000 in that period, the teams have promised to commit to playing "at least" 30 straight hours, or until they pass out, whichever comes sooner. And if the teams do manage to beat all three campaigns before 24 hours is up, they'll jump into Big Team Battle on Halo: Reach to take on the community.

You'll be able to watch all the action live on Ustream, and you can drop donations to the crew through their ChipIn page, which supports PayPal.

This article originally appeared on GamePro.com as Charity Halo Marathon