The Forgotten Tomb levels (or "onslaughts" as the game calls them) all share the same map and require you to survive 10 waves of attackers, that become more lethal and progressively harder to kill with each level. Levels 6 - 10 have a noticeably larger number of monsters for each wave.
Though Forgotten Tomb is marked as being for heroes of level 60 and greater, it is arguably easier than Hellfire . In large part it's because of the terrain, in particular the stairs, that you can use to control how you face your enemies.
You'll get a lot of loot, though not necessarily of great quality. You should have around 20 free slots if you don't want to the game to auto-sell the worst loot for you.
Forgotten Tomb is located in an ancient ruin, located in the middle of a desert, and you'll find lots of stone and sand. Be aware that the sand can sometimes hide spikes that will jut out at you. At level 1, the spikes will do less than 962 physical damage (fixme: probably much less!).
The safest place in the map is probably in the right corner. There's no sand, you're close to a Life Shrine, and more importantly enemies will have to traverse a set of stairs to reach you. You can use this to great advantage against the long-ranged foes, such as skeletons and mages, since their attacks will pass over your head or under your feet.
Map[]
The starting area is at the bottom of the map.
Wave 1[]
This is actually divided into 3 sub-waves. They all spawn in the starting area.
Sub Wave 1
3 zombies
Sub Wave 2
4 skeletons with 1-handed scythes.
Sub Wave 3
1 Anubis Warrior (not a mini-boss), some skeletons, and some zombies. At level 1 the Anubis goat will do more than 962 physical damage, so kill him first.
Wave 2[]
This has 2 sub-waves, which both spawn in the left corner of the map.
Sub Wave 1
3 skeletons with glowing maces spawn in the left corner of the map.
Sub Wave 2
3 skeletons with bows. No elemental damage.
Sub Wave 3
1 Elite Minotaur (mini-boss). At level 1 does less than 962 physical damage. Drops loot similar to level 2 (green) gems at level 1.
Wave 3[]
This has 2 sub-waves, which spawn from the top of the map.
Sub Wave 1
2 Fire Goat Mages and 1 big-armed zombie
Sub Wave 2
3 Fire Goat Mages and 1 Doom Spider (mini-boss). The Doom Spider will throw acid at you, but it does less than 456 poison damage at level 1. Drops loot similar to level 2 (green) gems at level 1.
Wave 4[]
This has 2 sub-waves, which also spawn from the top of the map.
Sub Wave 1
1 Elite Skeleton Archer (mini-boss, frost damage) and 3 big-armed zombies. The skeleton archer has a 3-frost arrow attack that will do less than 632 ice damage (fixme: maybe much less) at level 1. Drops loot similar to level 2 (green) gems at level 1.
Sub Wave 2
1 Temple Guardian and 1 Elite Anubis Warrior. At level 1 they will each do less than 962 physical damage. They drop loot similar to level 2 (green) gems at level 1.
Wave 5[]
This has only a single wave, which spawns from the left side of the map. You'll fight 1 Elite Skeleton Archer (mini-boss, does frost damage), 2 Doom Spiders, and 1 normal skeleton archer. As before expect to get loot similar to level 2 (green) gems at level 1.
Wave 6[]
Spawns in starting area, at the bottom of the map. You'll face 1 Elite Amnit Wizard (mini-boss, does frost damage) and 2 big-armed zombies.
Wave 7[]
Spawns in starting area as well. You'll face 1 Elite Minotaur Lord (mini-boss) and one skeleton archer.
Wave 8[]
Spawns in starting area. You'll face 1 Cobra Lord (mini-boss) and 2 Fire Goat Mages. The Cobra Lord is extremely fast and hits incredibly hard. Do not let it hit you! It can combo similar to your Dash attack, and if it completes its combo it might kill you outright, even with lots of armor. For example, at level 1 the Cobra Lord does far more than 962 damage with the first part of its Dash attack.
At level 1 expect level 2 (green) gems from killing him (yes, still level 2 gems).
Wave 9[]
Spawns from the top of the map. You'll have 1 Elite Skeleton Archer (mini-boss, does frost damage), 1 Elite Minotaur (mini-boss), and 1 Elite Amnit Wizard (mini-boss, frost). At level 1 they will all hit for less than 962 physical damage or 632 ice damage.
Wave 10[]
Spawns from the top of the map. You'll face 1 Cobra Lord (mini-boss) and 1 Risen Sacrifice (mini-boss, looks like Minotaur Lord). At level 1 the Risen Sacrifice does less than 962 physical damage.