The Last Wish raid, which comes as part of the Destiny 2 Forsaken DLC, contains three main bosses - of which one is Morgeth, the Spirekeeper - that players have to defeat before they can take on the final boss. First off, players need to reach at least level 550 and complete the Forsaken story campaign to even unlock the Dreaming City, which is where they will then launch the Last Wish raid from. That, plus obtaining select items as well as a fireteam capable of undertaking such a challenge are just as necessary in order to succeed.
Like in the many previous boss encounters in Destiny 2, Morgeth is immune to damage. So the only way to deal damage to him is to obtain Taken Strength, a buff that is only gained by jumping into the glowing white-and-black orbs; there are 10 surrounding the arena, but they don't all spawn at once. Some things to note regarding the Taken Strength orbs: players effectively steal the Taken Strength so that they inhibit Morgeth's ability to increase in strength. Otherwise, if Morgeth reaches 100%, he will wipe the fireteam. Also, since each player cannot hold more than two Taken Strength buffs at a time, they should be sure to split them up with the team.
The encounter starts when a player jumps through the first Taken Strength orb field in front of Morgeth. From there, there will be three waves; four Taken Strength orbs will spawn in each of the first two waves with the last orb of Taken Strength spawning in the final wave. It's only after receiving the 10th Taken Strength buff that players can start to deal damage to the boss. Furthermore, defeating the Eye of Riven will drop two orbs of Taken Essence, but only two can be in the arena at any given time.
Throughout the boss fight, if a player has one or two buffs of Taken Strength, there is a chance that he/she will be placed into Umbral Enervation, a field that traps players in place for up to 20 seconds. It's vital that another player free them from that stasis by using one of the orbs of Taken Essence (use the grenade ability to free them). If the player isn't freed by the time the counter runs out, they will die. By freeing the player, the person using the Taken Essence will absorb the Taken Strength from the player in stasis. Remember, players cannot stack more than 2 Taken Strength at a time, so determining a strategy to properly distribute the Taken Strength buffs throughout the encounter is crucial to winning.
Once the damage phase begins, players should attack Morgeth with everything they've got, but it's important to always keep an eye on his strength percentage. It's best to use the Taken Essence bomb (by using the super ability of whichever player holds the Taken Essence) right when Morgeth reaches 80% to 90% in strength, so that the encounter will reset. At that point, repeat the aforementioned process, but note that the number of Taken Strength will increase. Players will start with two Taken Strength fields in the middle instead of one. Do this until he dies.