The 7th Heaven is one of Toan's swords in Dark Cloud, and one of Monica Raybrandt's swords in Dark Cloud 2. In the first Dark Cloud, this sword is built up from Dark Cloud, but in Dark Cloud 2, this sword is only built up from the Lamb's Sword. It doesn't come with any special abilities.
Its in-game description reads "Beautifully ornate sword. He who sees it is captivated."
This sword is, like the Sword of Zeus a penultimate weapon, so its Attack stat will go up faster (+3 per level-up instead of +2), it also has some high elemental stats. So it shouldn't be a problem to defeat enemies with this sword. It also has the slow 5-hit combo, much like the Long Sword so you'll attack at reasonable speed.
You can perhaps find the 7th Heaven in dungeons' chests : you can't invent this weapon and you can't buy it.
It's a high-tier weapon (Tier 8) : it'll gain 6 Synthesis Points (SP) per level-up.
7th Heaven originated in the original Dark Cloud, and was one of the ultimate weapons in the game.
The 7th Heaven can only be built up into the Island King when it reaches the following stats : 135 in Attack and 180 in Smash, Exorcism, Beast and Scale. You must also defeat at least once a Lancer, a S. Club and a S. Joker.
Weapon Hit Points : 92
Attack : 123
Durable : 85
Flame : 0
Chill : 0
Lightning : 0
Cyclone : 125
Smash : 125
Exorcism : 125
Beast : 125
Scale : 125
Synthesis Points : 2