UPDATE: Gumi updated the ingame news. The challenge reward remains as the title stated.

Haven't seen this mentioned anywhere yet, but according to both the datadump and the wiki, Oblivion, the Sora-only weapon, is currently the main mission completion award, rather than the "Under 5 party member" bonus reward as originally advertised in the news. Instead, the "5 or under" is the TMR moogle award.

Thought you ought to know BEFORE the challenge event goes live, especially how the news post had people talking about ignoring the 5-man challenge.

^{Edit: Sigh, almost immediately after the creation of this post, an unknown user changed the wiki to reflect what was advertised, rather than dumped. Changing the wiki link I use to the original post-dump revision.}

"8630101": {
    "dungeon_id": 86301,
    "name": "Heartless: Darkside",
    "type": "BATTLE",
    ...
    "challenges": [
        {
            "string": "Complete the quest",
            "parsed": ["Complete the quest"],
            "reward": ["EQUIP", 302008500, 1]
            /* Oblivion (KH) */
        },
        {
            "string": "Use a limit burst",
            "parsed": ["Use a limit burst"],
            "reward": ["ITEM", 106307100, 10000]
            /* 10,000 Guard Armor Raid Coin */
        },
        {
            "string": "Defeat the darkside with magic",
            "parsed": ["Defeat Darkside with magic"],
            "reward": ["KEYITEM", 50, 2]
            /* 2 Rare Summon Tickets (Couldn't find datadump ID, so took wiki's word for it)*/
        },
        {
            "string": "Party of 5 or less (Companion included)",
            "parsed": ["Party of 5 or less (Companion included)"],
            "reward": ["UNIT", 904000104, 1]
            /* 4-Star Trust Moogle  */
        }
    ]
},