Is it kinda' like the Torque Bow? Aim. Charge. Fire. Upon detonation it sends out hard-light shrapnel that detonates once again.
You're on the right track. For now I'm calling it the
Crossbolt and it's designed to be the mother-of-all-weapons and hearken back to the days of Doom and Quake. Chief would get it in campaign as an eleventh-hour-superpower. It's balanced more appropriately for multiplayer, however.
I decided to make the story mechanics as ridiculous as possible because I feel like there aren't enough ridiculous story-based weapons in Halo. To put it simply, the Crossbolt is meant specifically to wipe organic matter off the face of time and space, especially in the case of the Flood, where it's designed to:
1. Cut off all neural links to the Gravemind.
2. Destroy any and all organic tissue.
3. Eliminate the target in all present forms of space-time: visible three-dimensional space, Slipspace, Denial of Locale, Natal Void, Shunspace, Trick Geodetics, and the Glow, an entirely photon-based dimension.
See "Technology" subsection for more details.
In other words, the Crossbolt is operating in about 9 different dimensions at once, which are condensed into five primary domains: Neural, Armour, Aether (Photonic), Slipstream, and Geometrics.
These five primary domains are the reason for the slow charging and firing speeds of the Crossbolt; it requires a huge amount of processing power to register the gun and the projectile in all these different dimensions. The effect, however, literally wipes any and all traces that organic might have had from existence.
The Crossbolt takes five seconds to charge, much like a Spartan laser - the animation consists of the Spartan aiming and using their hand to count to five on their fingers, and then clench their hand into a fist to fire the "arrow" or spear. The hardlight arrow travels at an arc, and is incredibly slow; it goes at roughly the pentuple root of the speed of light, or
miles an hour.
You can outrun the projectile in most circumstances.
To compensate, the arrow can bounce off the floor at least once regardless of distance, and the arrow itself is about five feet long - and the entire spear has a hitbox, and touching it anywhere will activate it.
If the arrow
does manage to make contact, however, it's an instant kill with near-infinite potential to keep killing. The target is speared by the arrow and begins "derezzing" like a Promethean, but blue instead of orange. Additionally, five "doppelgangers" phase out of the Spartan's embers and all crumple over in a death throe - Neural's Spartan is a bunch of white pixelly dots like the loading screens in 4. Armour's spartan is literally the Spartan's armor without any undersuit below it, as well as no coloring or textures - it's just the white 3D model. Aether (Photonic) is the texturing and coloring of the armor. Slipstream is a glow effect like Damage Boost or Speed Boost, with no Spartan underneath, and purple. Geometrics is essentially a wireframe model of a Spartan.
These five "doppelgangers," their death throes combined, have the radius of roughly a rocket launcher explosion - and if any of them come into contact with another Spartan, the process repeats again. This makes the Crossbolt absolutely lethal against enemies huddled together in corridors or bases. It's also worth noting the Crossbolt projectile will only affect Spartans - it literally phases right through vehicles without doing any damage, making it an ideal killer if you want to keep the vehicle itself intact.
This thing would be a monster against Flood in close quarters too, of course. It would probably only have about five shots on a pickup, but unlimited ammo in Campaign.
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