If you have an ongoing paragon- or epic-level D&D game going, you're probably get a lot of mileage (both literal and figurative) out of the linked portal ritual and its higher-level alternatives. They're bread-and-butter transportation for any sort of world-spanning campaign.
Some tips to make your portal rituals work better at the D&D table:
The rules are silent on this issue, but if you're the DM, you should decide for yourself what people who observe the linked portal ritual learn about the destination of the teleport. Whether it's PCs chasing the Big Bad Evil Guy who portals away or NPC bounty hunters trying to track down the PCs, your table will eventually confront the "Do we know where they went?" question.
The rules specify that the sigils in the temporary circle at the origin point (or the modifications to the permanent circle at the origin) disappear at the end of the ritual. But that's usually a few rounds after the portal users step through, so anyone close enough (how close? I'd say six squares or cobble up a quick Perception-based answer) can see the sigils, and thus be able to duplicate the ritual and arrive at the same place.
More interestingly, I'd also rule that anyone who listens to the ritual being performed hears the sigil combination and thus can duplicate the destination. (But you have to know the linked portal ritual yourself, or all those words are just a bunch of gibberish to you.) Off the top of my head, I'd say that the sigil combination is repeated roughly every minute during the 10-minute ritual. If you make it fairly easy for people to follow each other with teleports, you open up more interesting plots than you close off.
The rules are also silent on the reverse engineering point, but I'd rule that you can't learn where a destination is by merely knowing the sigil combination. In other words, you can't determine that the sigil combination "aluk-tam-yesh-quor-tahn-tahn-galli-prao" will send you to a jungle ziggurat in southern Qbarra. It's probably more fun and more adventure-enabling to make the PCs perform the ritual and check it out firsthand. At my table, you could probably talk me into an arbitarily high Arcana check, but unless you're scraping the top of the epic tier, your odds won't be good.
Use "Real" Sigils!
One way you can really make the portal rituals come to life at your D&D table is by making your players keep an "address book" of the sigil combinations they know. Rather than just saying, "you discover the sigil combination for Castle Scarypants," tell your players, "You discover that maur-yao-kuru-trehn-plau-ganou-rikh-shen is the sigil combination for Castle Scarypants." Keep a record for yourself, of course. You'll know the effort pays off when one player says to another, "What are the sigils for Castle Scarypants? We can go there!" and someone else says, "Just a sec...it's maur-yao-kuru-trehn-plau-panou-rikh-shen."
I used a variation of this myself in my epic-level post-apocalyptic Eberron game. I used the English alphabet for the sigils, though, which in retrospect didn't really seem fantasy enough. But my players dutifully kept track of where "M-Y-K-T-P-P-R-S" would take them, and whenever they found a bad-guy library, one of the first things they'd do is ransack the place looking for sigil combinations.
So there's potential there. But to really come to life, the portal sigils deserve a fantasy alphabet of their very own--and it needs to be different from the existing D&D alphabets because presumably everyone uses the same sigil "language." Twenty or thirty characters is plenty, and each sigil combination should be six to eight letters long.
Wizards R&D is a busy place, but wouldn't it be great if they'd come up with a portal-sigil alphabet and release it as a font? Then we could all make cool handouts for our players.
Languages and Rituals
More broadly, the Player's Handbook is silent on what language a ritual is performed in. The language doesn't matter in most cases, although it would be a little weird if assistants to the ritual don't share a language with the lead ritualist. We imagine rituals are complicated things, and it probably bends suspension of disbelief to imagine that you can pantomime ritual instructions effectively. You ain't that good at charades, Baron E'Ville. And language matters also in the fairly common case when PCs come across an in-progress ritual and want to know what the ritual is before they (inevitably) interrupt it.
My answer is that rituals have no language per se--they use a specific vocabulary from the relevant skill (Arcana, Religion, etc.). You can't conjugate a sentence in Arcana-speak, but you can evoke a great deal of magical power if you say the right Arcana words and throw around the right alchemical reagents. That answer is a bit of a reach, but easy overhearing of rituals is probably more adventure-enabling than it is adventure-thwarting.
Things I'd Do Differently
If I had another chance to make changes to the linked portal ritual, here's what I'd do:
• I'd make the default diameter of the portal circles 15 feet. That lets five PCs use it at the same time, and it's a lot friendlier to monsters. And I'd explicitly make it clear that big dudes can make even bigger portal circles so they don't have to squeeze through the little ones. (If the Huge titan arrives at a smaller portal circle in a constrained space, he takes his lumps, of course.)
• I'd have the portal sigils remain for, say, 1d6 x 5 minutes after the portal closes. (They'd be visible, but powerless at that point.) That makes it even easier for the pursuer to keep the chase alive. Keeping the chase alive good for business, most of the time.
Out of Context: The Ice Stone has melted!
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