This includes an upgrade of the Orion's Prophecies trophy on your userlookup, now to a maximum of gold if you have completed the previous Orion quests in the Festival of Neggs, Altador Cup, and Faerie Festival. Otherwise, you will get a relevant colour upgrade based on the number of Orion quests you completed.
There are also a number of item prizes awarded, the number of which depends on the amount of Orion quests you have previously completed.
Note that these prizes are meant to be displayed in a popup after clicking through the dialogue, but this does not trigger for all users. However, even if you do not get the popup, the items are correctly placed in your inventory and the trophy correctly upgraded.
IMPORTANT: This is NOT the advent prize for today. In order to qualify for the Winter Site Theme by collecting prizes on each day of the advent calendar you must also click the collect prize button under the comic today. There is an additional Premium item prize, but for non-Premium users, the daily prize is Neopoints.
There is also an additional quirk today in that the comic is not the same for everyone. There are four versions of the comic, randomly displayed to users, which show a different part of the tablet at the bottom:
Thanks to Neopets user caraotico, who compiled the four parts into a single image:
From the completed tablet, it appears we may be heading to Tyrannia in a future Orion event.
Also note that this is NOT the end of the Orion content in the Advent Calendar. The event's FAQ specifies that there will be additional storyline related days on December 29th (tomorrow!) and December 31st.