The ESRB page for Halo 5 lists three content descriptors: "Blood," "Mild Language" and "Violence." Here's the full rating summary:
This is a first-person shooter in which players assume the role of a super soldier (Locke) searching for a missing character. Players use pistols, machine guns, grenade launchers, and futuristic weapons to kill alien and human enemies in frenetic combat. Battles are highlighted by realistic gunfire, explosions, and occasional blood-splatter effects. Characters can also use "assassinations" to kill characters by snapping their necks, or by stabbing them with bladed weapons. The word "a*s" appears in the dialogue, as well as occasional taunts/insults (e.g., "I have copulated...with your genetic progenitors!"; 'Your father was a filthy colo and your mother was a hole in the wall!').
It's not apparent how Halo 5 managed to sneak in under the T rating guidelines while all the other games were rated M, especially since this game contains the assassination feature that appeared in Halo: Reach and Halo 4. But you might find a clue if you follow the trail of blood.
Halo: Combat Evolved, Halo 2 and Halo 3 were all rated for "Blood and Gore," which would be more likely to lead to an M rating than just "Blood." The ESRB website doesn't list summaries for those older titles, but here are excerpts from the summaries of later Halo games.
Halo 3: ODST: "Puffs of blood are emitted from injured aliens/humans, and blood is sometimes smeared on walls and on the ground."
Halo: Reach: "Human and alien characters emit spurts of blood when injured; players are able to shoot dead enemies, causing more blood to splash out. Blood is sometimes depicted on the walls and ground."
Halo: Combat Evolved Anniversary: "Frequent blood-splatter effects occur when enemies are shot, sometimes staining the surrounding environment; aliens often break into bloody fragments when killed."
Halo 4: "Large blood-splatter effects occur when humans are shot; some sequences depict bloodstained environments."
Contrast those descriptions with the "occasional blood-splatter effects" listed for Halo 5, and the game's T rating starts to make sense. There's no indication if developer 343 Industries or publisher Microsoft Studios toned down the blood or other elements of Halo 5 with the intention of securing a T rating rather than an M rating; we've reached out to the companies for comment, and will update this article with any information we receive.