SPV3 Halo PC Mod

Sorry about the snotty response.

To answer your original question, here's a more detailed explanation from ZerueLX11:

"SPV3 is the culmination of a vision that was started over 10 years ago, to revamp the original Halo campaign with features and ideas 
from later games, expanded fiction, and cut content originally envisioned for Halo CE. With a strong emphasis of preserving the 
addictive and tuned gameplay of the original, and a stellar Legendary difficulty, SPV3 focuses extensively on retaining that Halo CE
feel.
 
 
SPV3 makes use of Open Sauce, a community-made game extension that enables many new features, from upgraded visuals to new gameplay mechanics. Open Sauce is required to play SPV3 the map can't exist without it. Open Sauce can be found both on our subreddit 
( www.reddit.com/r/halospv3 ) as well as bundled with your SPV3 mod content download.
 
Open Sauce 4.0 requires a properly-installed copy of Halo Custom Edition, updated to 1.10.  See the installation guide if you don't 
know how to get these.
 
We highly suggest all players, no matter how experienced they are begin the game with Pillar of Autumn, as there are so many new 
features and additions. Each level introduces you to new abilities, weapons, enemies and vehicles, and you will be overwhelmed simply
jumping into what may have been your previously favorite map. We also highly suggest you not play Noble Difficulty until completing
SPV3 on Legendary, as it is a new even harder experience that will test even the most seasoned Halo and SPV3 players."