Finishing the scenario will also give you the Zandalari Direhorn mount to use for all your Horde characters.
If you want to make an Allied Race like the Zandalari trolls, you will need to have a level 110+ character on the server you are making a character on, so it's not good for complete rerolls on new servers.
There are three hidden requirements that you need to complete before you can begin the recruitment scenario to unlock the Zandalari trolls. The quest to begin the recruitment scenario will simply not show up until these three requirements have been met, and all of them have their own achievement that is used for tracking the progress. The three requirements are as follows:
Step 1) Earn Exalted with the Zandalari Empire (achievement Zandalari Empire)The Zandalari Empire is a horde-only faction found in Zandalar. The main way to get reputation with them is to complete the main story line in Zuldazar, which will get you to Honored reputation. After this point you should focus on doing World Quest in this zone to get more rep. You can also do missions from your mission table to get some extra reputation with them as well.
It will take a little while to get from Neutral to Exalted, since you will need to do lots of World Quests for many days before you reach Exalted.
You can also use the item Contract: Zandalari Empire to get an extra 10 rep with them any time you complete a World Quest (even those who don't reward Zandalari Empire rep in the first place). This item will give you a buff that lasts for an entire week, so pick up a new one after the week has ended. Scribes can craft this contract, but you can also buy it from the Auction House.
Step 2) Complete the main quest lines in Zandalar (Zandalar Forever!)The achievement Zandalar Forever! requires the player to complete the main story line in Zuldazar, Nazmir, and Vol'dun, as well as as the two scenarios Blood Gate and Zandalar Forever. These scenarios are available after you have completed the three main story lines, and does not take very long, so this part of the unlocking should be somewhat fast and easy for most players.
Step 3) Complete the Tides of Vengeance War Campaign (Tides of Vengeance)The War Campaign continues with the Tides of Vengeance patch, with the forth and final part of the releasing on January 22 2019. This quest chain is pretty straight forward, but there are two hurdles you need to overcome to finish it:
1) You first need to complete the 8.0 War Campaign. The Tides of Vengeance War Campaign is a direct follow-up from the original War Campaign, so new players have a long quest chain ahead of them if they haven't done parts of it already.
3) You need to get Exalted with the Honorbound. The reason behind this is that all parts of the War Campaign is gated behind reputation with The Honorbound, and the final part require you to be Exalted with them to get the last quest. This means that you need to grind a lot of reputation with them if you want to finish your Tides of Vengeance War Campaign. It's luckily one of the easiest factions to get reputation with, and all World Quests in Warfront Zones and in all Kul Tiras zones gives reputation with them. Contributing with materials for the Warfronts also give a decent amount of reputation with them. A final way to get some extra reputation with them is to do Faction Assaults that happens every 19 hours, and completing the World Quests as well as the main quest there gives a good amount of extra reputation.
Step 4) Do the Recruitment ScenarioA new quest chain will pop up for you after you have completed all the three requirements listed above, which will end with the Recruitment Scenario and unlocking of the Zandalari trolls. This quest chain and scenario also takes a little while to finish, but you should easily be able to do it within a few hours.
There is not really a lot to say about this part of the chain, other than just follow the quest chain to get it done. Once you complete the scenario, go to Orgrimmar and deliver the final quest to get your mount and unlock the Zandalari trolls as a playable Allied Race.