This guide will tell you how to tame a number of creatures from Battle for Azeroth zones that are normally friendly. The trick involves a toy: N'lyeth, Sliver of N'Zoth. This trick was first reported on our forum in Feb 2020, and was used by the Secret Finder Discord to discover how to tame some unique cloud serpents.
This trick is only for friendly creatures in Battle for Azeroth areas, so: Zandalar, Kul Tiras, Mechagon, Nazjatar, and the assault phases of Uldum and the Vale of Eternal Blossoms. The creatures may also be friendly to one faction but already-tameable by the other. It also requires the creatures to be flagged tameable, which most friendly creatures aren't. The few that are are normally not intentionally flagged, but none of the ones we mention here are disruptive to the game if tamed.
The toy, N'lyeth, Sliver of N'Zoth, requires PvP, and it's not easy to obtain. We recommend you get friends to help you, to provide targets for killing. We'll provide some information below, but we also recommend you check out the comments in the Wowhead link for much more detailed instructions. Also, check out the Rosepetal Ranch Discord community who generously help players to obtain their N'lyeth toy.
Firstly: turn War Mode on (in your capital city). Then proceed to a Battle for Azeroth area - for reasons given below it is recommended you choose Nazjatar.
You must then kill 10 members of the opposite faction without dying. It is also important that you not hearth, take a portal or leave Battle for Azeroth zones or your counter will reset. If you are using friends to help you achieve this goal, there's a good chance they'll end up in different phases to you, but this doesn't seem to be such an issue in Nazjatar. (One of two reasons that Nazjatar is recommended.)
As soon as you kill 10 members of the opposite faction without dying, N'Zoth offers you a quest: Servant of N'Zoth / Servant of N'Zoth to kill 10 members of your own faction without dying. He also gives you a temporary, one-time version of N'lyeth that will place a buff on you that makes you hostile to your own faction. It is important that you not hearth, take a portal or leave Battle for Azeroth zones, or the quest will be removed entirely and you'll have to start over from scratch. It is also important that you complete this part within 60 minutes of using N'lyeth or the quest will be removed. Note that when you kill a character they receive a 10-minute debuff that prevents them from counting for your total a second time, and the debuff timer only counts down while the character is online. So if you're using friends you'll need the help of more than one, unless one friend can bring 10 level 50 characters for you to kill, or the debuff timer will prolong the task for more than 60 minutes. To further complicate the situation, it is likely the game will place a bounty on your head after you've killed one or two of your own faction. This bounty will flag you on the map and could well attract members of the opposite faction to try to kill you. A popular suggestion in the Wowhead comments is to hide out in Mardivas's Laboratory in Nazjatar, as this cavern is phased and nobody else can reach you. However, neither can your same-faction targets, so if you're using friends you'll have to get them to queue up outside (and a little back from the cave entrance) so you can quickly run out and kill them then run back inside to safety. To make this easier for you (and less costly for your friends), they should ensure they've removed all their armour.
Assuming you succeeded, you'll receive the toy on your tenth kill. Congrats! While not related to the objectives here, it's worth noting that killing your own faction actually counts for the The Alliance Slayer / The Horde Slayer quests and title, as if they were players of the opposing faction, so you might consider killing a couple more at the end just to complete that.
For most creatures: just turn War Mode on, proceed to the creature, activate N'lyeth, Sliver of N'Zoth and tame away! Note that the toy has a one-hour cooldown, so it's safest not to use it until you need to. Its effects are also lost if you ever leave Battle for Azeroth areas, hearth, or take a portal, so plan accordingly.
The Ivory Cloud Serpent is a little more complicated. Like all cloud serpent tames, unless you're a Pandaren you must purchase and learn the tome "How to School Your Serpent", from San Redscale at The Arboretum in The Jade Forest. The resulting skill is account-wide and the tome can be purchased and learned by any character on your account, of any faction, that is Exalted with the Order of the Cloud Serpent. You must also have turned War Mode on.
The Ivory Cloud Serpent spawns once every ~2-3 hours in the Vale of Eternal Blossoms, but only on days when a Mogu assault is in progress. This only happens every second week, for a few days of that week. It won't appear at all on weeks when a Black Empire assault is happening. It's also best to pick a time of day when nobody else is camping it, or players that are trying to lasso it for the mount will get it before you can.
You'll need to wait for the Ivory Serpent to spawn just a little south-west of the camp and bonfire at Winterbough Glade - coordinates approximately 27,55. Don't use the N'lyeth toy yet! While you're waiting be mindful that PvP players may try to kill you. If you're too high in the air you'll receive slowing debuffs from the Great Worms From Beyond flying overhead, or they may run into you and kill you. If you're too low, PvP players will be able to attack you more easily. A rare-detecting add-on, such as Silver Dragon, can be of use in warning you that the serpent has spawned.
When the serpent spawns it will start flying rapidly to the south-west and perform a roughly clockwise circle of the Vale. You shouldn't try to follow it, but instead fly directly west, to the top of the Gate of the Setting Sun. Land on the ridge on top of the Gate, just a few tiles north of the southern end, and face south and wait for the Ivory Serpent to approach. If you were quick you should arrive with some time to spare. Drop a Freezing Trap along the ridge, closer to the south. For good measure drop a Tar Trap even closer to the edge to give you more time to react. As soon as the Serpent nears the Gate, and before it reaches the traps, activate your N'lyeth toy making you hostile to it. As soon as it hits the Freezing Trap, tame away and hope that nobody arrives and tries to attack you. Good luck!