This is after you have made your way through the Forerunner wall and commandeered the Gauss Warthog. Take the Warthog down the dusty path and complete the following battles with Brute Choppers, Ghosts and Wraiths. As you approach the bottom of the slope, there will be a few Brutes, and depending on what difficulty you are on, will usually have Fuel Rod Guns, Brute Shots and Bubble Shields. As you are fighting them, a few more Brute Choppers will be present behind them, and after a short while the Scarab will make its way down to the battle.
When you get to the bottom of the dusty slopes, there will be another small structure in the middle of a flat surface. Sometimes a few Grunts and Brutes may be in this structure with Fuel Rod guns. This is the same flat surface that the Scarab will appear on. After defeating the Scarab, a structure to your right has a slope on it, and you will need to fight your way to the top, past Brutes, Grunts and a few Jackals. After defeating the enemies, a Pelican dropship will come down to drop off allies, including the Arbiter.
Right up the top, instead of entering the building with 343 Guilty Spark, look to the right, past the Pelican that drops off the Arbiter, and you will see a pathway. It may not be too noticeable at first, so look carefully. Follow the pathway to the end, and you should find a blood splatter on the wall (or floor).
- It may have been put there for other uses, and later forgotten.
- It is possible that it was just a detailed part of the Campaign map.
- It is impossible to find this Easter Egg on Co-op off of Xbox Live, given that both blood splatters and bullet holes do not appear on splitscreen co-op.