A Town Portal trapped by An Evil Force.

Bone Prison is a skill used by Diablo to enclose Town Portals, and trap your character. Although it has the same name, it is not the same skill as the Necromancer's Bone Prison.

When Bone Prison is cast, your Town Portal will be ensnared by several long, curved bone spires that look like giant ribs or tusks. They will be called "An Evil Force" if you hold your mouse over them.

Diablo is capable of casting Bone Prison from quite a long distance, so you may find your Portals trapped even if you have been fighting the Lord of Terror a screen or more away. Diablo often won't follow you that far if you run from him, so you may still be able to smash the evil force without immediate risk, however, this can still be quite dangerous as you probably ran because you were about to die. To play it safe, you can open Town Portals very far from the battle zone, such as near one of the five seals. You can also simply open another Town Portal, if you have a tome or scrolls. However, these methods are useless if you are the one trapped. At this time, the only reasonable safe method for escaping from one is to destroy it as fast as possible, or, if you're lucky, open a Town Portal inside* (Note, however, that as Town Portals return you to the same place, and the "Evil Force" has a very long duration (It is unconfirmed if it expires, due to the fact that most players stuck in one were more concerned about escaping before they died), it is safest to return through an alternate method, such as a Waypoint).

In Diablo III, this skill summons several Prisons, one by one, and instead of blocking a portal, it imprisons the player, allowing Diablo to teleport to them and drain a huge portion of their Life. Aside from him, this skill is also used by Inquisitor Sebacious.

Damaging Skills
FireFire ArrowFireballFire BoltFirestormFire WallFlame CircleFlaming SwordHydraIncineration NovaInfernoMeteor
IceArctic BlastBlizzardCold ChargeCold TouchFreezing ChargeGlacial SpikeHoarfrost
PoisonPoison BoltPoison BreathPoison CloudPoison SpitPoison SprayPoison StingPoison Strike
LightningLightningLightning ArrowLightning GlobeRed Lightning HoseShocking HitSpark Defense
PhysicalBlow DartChargeCorpse CannonDestructive StrikeGround TentacleLeapLeech-life BallQuill Dart
MagicBone SpiritHellish ProjectileUnholy Bolt
Non-damaging Skills
Indirect Attack — Blood ManaCursesMana RiftMinion FrenzyPutrifySpin WebStun
Spawning — Bone PrisonFestering AppendagesLay EggReviveSummon Minion
Self-effect — Bone ArmorBurrowFlyFrenzyHealTeleportVile Effigy
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