Rifts Ultimate Edition, page 122 wrote:The Dimensional Travel ability can target the Shifter's home dimension, a random dimension, or a dimension visited by the Shifter at some time in the past.
When targeting a random dimension, the Rift can open anywhere within the resulting dimension. However, portals are drawn to high P.P.E. places like planets rich in P.P.E., so it is exceedingly rare that a random portal will appear in the depths of space or on a completely lifeless planet. However, it could lead to a place like Rifts® WormwoodTM which is inhospitable and dangerous to humans.
Targeting one's home dimension will create a Rift on a world the Shifter has visited previously, but the portal will open at a random but viable location on that planet. A human Shifter targeting Rifts Earth will not end up in the ocean, but on a landmass, and not in the middle of a desert either. Mid- to high-level Shifters (levels 5-9), can target a specific country or continent, and the highest level Shifters (level 10 and above) can arrive within 50 miles of their desired destination. Targeting a world previously visited in another dimension means the Shifter (and any passengers) will arrive at a reasonably hospitable, but random location on the desired planet.
For greater accuracy and for two-way travel, Dimensional Portal (1,000 P.P.E) or Re-Open Gateway (180 P.P.E., but can only re-open a portal to wherever the Rift went to last) are recommended.
Other books have included more details on the Dimensional Portal spell, but that info has since been lost in the endless cycle of copy/paste from book to book. Beyond the Supernatural 1st Edition had the table to determine what creature(s) may come through the portal and how often to roll on it. Transdimensional TMNT has excellent notes on how to direct a Dimensional Portal to reach specific dimensions.
I think if you combine all this information you have more of a complete spell which could look something like this:
Dimensional Portal
Range: A few feet away.
Duration: 30 seconds per level of the spell caster, or one minute (4 melees) per level of experience when performed as a ritual.
Saving Throw: None.
P.P.E.: One Thousand.
The dimensional portal invocation opens a two-way door to another dimension. The mage can open a door to a specific world or to a random destination. For random dimensions, use the tables in Rifts Ultimate Edition (pages 196-197) or Rifts Dimension Book 7 (pages 24-26) to determine where the portal opens. Portals are drawn to high P.P.E. places like planets rich in P.P.E., so it is exceedingly rare that a random portal will appear in the depths of space or on a completely lifeless planet.
The spell caster can concentrate on any of the following things to direct a Dimensional Portal:
Pursuit Focus. This is useful for following someone lost or moving between various dimensions. The spell caster concentrates on the specific person or creature he is seeking, calling them by name. If well known to the spell caster, there is a +10% chance of success. If the spell caster is working from just a description, or picture, or from just a brief meeting, there is a -15% chance of success. A successful portal requires a roll below 20% + 5% per level of the spell caster. If the roll fails, roll again on the following table:01-15%: A portal opens to the person's home world.
16-30%: Portal opens to a dimension previously visited by the person.
31-50%: Portal opens to a close parallel to the person's home world.
51-00%: Portal opens to a random dimension.
Object Focus. The spell caster touches any object or person that originates from another dimension. Chance of successful location of the object's home dimension is 10% + 2% per level of experience of the spell caster. Failure means that a portal is opened, but to a completely random dimension.
Historical Focus. A spell caster can attempt to open a portal to a dimension with an alternate history. To do this with any accuracy, the spell caster must have a skill in History that covers the pivot point where that dimension became different from our own. Chance of successful location requires two rolls. The first must be under the spell caster's history skill, and the second is 20% + 4% per level of experience of the spell caster. Failure on either roll means that a portal is opened, but to a completely random dimension.
Inhabitant Focus. By visualizing the dominant race of the dimension, the spell caster attempts to find a new dimension. For example, the spell caster might try imagining a race of giant mutant pigeons. Chance of successful location is 5% + 1% per level of experience of the spell caster. Failure means opening a portal to a random dimension.
This spell is the only way a greater supernatural being can enter into our dimension. Once the dimensional portal closes, the only way back in or out is to open another portal. One of the real dangers of using this magic is that some "thing" unwanted often slips through. For every minute (4 melees) that the portal is open roll on the Quick Roll Monsters random generation table or roll on the following table to see what type of supernatural creature enters into our world through the portal:01-02% Angel
03-09% Entity
10-16% Lesser Demon
17-23% Greater Demon
24-28% Deevil
29-31% Greater Deevil
32-36% Gargoyle
37-43% Elemental Fragment
44-50% Minor Elemental
51-57% Major Elemental
58-64% Monster
65-71% Oni One Hundred or Chinese Demon
72-78% Undead
79-85% Daemonix
86-90% Lesser Chaos Demon or Lesser Russian Demon
91-94% Greater Chaos Demon or Greater Russian Demons/Spirit
95-99% World Slayer or Chinese Demon
100% Alien Intelligence Essence Fragment
Expanded creature tables for each type above are found in Rifts: Dimension Book 7 (pages 57-60). Once through the portal, the creature(s) is likely to take off and hide until it can get its bearings and formulate a plan of action. It will confront the summoning mage and his associates only if attacked or if it believes it can defeat him/them.