EDIT: References provided at the bottom.

The short answer is nemetic. Longer answer follows.

In English, nouns of Greek origin that end in -esis regularly form corresponding adjectives that end in -etic: antithesis, antithetic; diuresis, diuretic; emesis, emetic; genesis, genetic; kinesis, kinetic; mimesis, mimetic; synthesis, synthetic; tmesis, tmetic.

Although this is perfectly normal English derivational morphology to coin new words out of existing classical forms, usually the related words already had related terms in that form in the original classical languages. For example, Latin has synthesis and syntheticus, and Greek σύνθεσις and συνθετικός; kinesis and kinetic are from Greek κίνησις and κῑνητικός.

Sometimes closely related words exist. The noun tmema is related to tmesis and tmetic; the noun thema, themata is allied with thesis, as is anathema (although that’s right from Greek ἀνάθεμα).

English also likes to elaborate the -ic forms to create new nouns via -ics, as in genetics, and sometimes also use the -ic form to double up the adjectival endings by adding -al to the existing -ic to form adjectives and nouns, as in hypothetical, hypotheticals.

So the adjective related to nemesis must necessarily be nemetic. Interestingly, the semi-lookalike Nimetic™ exists as an expired trademark for a bonding substance used in dental reconstruction.

The OED doesn’t cover proper nouns like that, and ∗nimesis is not attested in the OED. It also doesn’t list nemetic. However, the related terms nemesism and its derivative, nemesistic, both are. Nemesism is ‘frustration and aggression directed against oneself’, and nemesistic is the adjective that corresponds to nemesism, not to nemesis.

If for some odd reason you don’t care to use that natural adjectival form nemetic, then you’ll have to return to the definition of nemesis and choose a synonym, then rederive a new adjective from that. The OED attests three primary senses of nemesis:

1. a. Usu. in form Nemesis. Originally in classical mythology: the goddess of retribution or vengeance, who reverses excessive good fortune, checks presumption, and punishes wrongdoing; (hence) a person who or thing which avenges, punishes, or brings about someone’s downfall; an agent of retribution. [citations from 1542—1989]

   b. orig. and chiefly N. Amer. In extended use: a persistent tormentor; a long-standing rival, an arch-enemy. [citations from 1933—2000]

2. a. Usu. in form Nemesis. Retributive justice; (also) an instance of this; the downfall brought by it. [citations from 1597—1989]

   b. An unavoidable consequence of (or occas. for) a specified activity or behaviour; an inevitable penalty or price. [citations from 1863—1991]

3. Astron. In form Nemesis. A small, faint star postulated as a companion to the sun in a hypothesis to explain the supposed cyclical nature of terrestrial mass extinctions.
   According to the hypothesis the companion star follows a highly eccentric orbit around the sun which causes it to disturb the Oort cloud every 26 million years, sending a shower of comets into the inner solar system. [citations from 1984—1994]

If you were looking for an adjective corresponding to sense 1b, you might choose the synonym arch-enemy, whence one could perhaps offer up the admittedly awkward arch-inimical. The word inimical comes ot us from Latin inimīcālis, itself a compound formed by combining in- not with amīcus friend. In modern parlance, one might think of that as an unfriend; indeed, enemy has the same origin, albeit via a more circuitous route, having passed to us from Old French enemi, anemi, anemy; compare also modern French ennemi, Provençal enemie, Catalan enemig, Spanish enemigo, Portuguese inimigo, Italian nemico.

One can use enemy as an adjective: enemy agent, enemy nation, enemy ships; Orwell wrote ‘Koestler‥was once again thrown into prison as an enemy alien.’ Used in this way it is stronger than merely unfriendly, taking on more of the added connotation of some specific adversary.

However, since you asked for a non-person, that seems to require sense 2b, not 1b. I’m not sure you want to go back to unfriendly or adversarial here, as the former is too general, while the latter has judicial connotations you may wish to avoid.

Really, I think I’d just go back to my original suggestion of nemetic. If that feels too bizarre to you, than perhaps you could try using nemesis attributively. But nemetic is certainly the natural adjective, so you might as well use it. Other people have certainly done so.

Citations

• 1972, Scott Gordon, Two Monetary Inquiries in Great Britain: The MacMillan Committee of 1931 and the Radcliffe Committee of 1959; Journal of Money, Credit and Banking, Vol. 4, No. 4 (Nov., 1972), page 966 ― There were, however, strong moralistic overtones to the arguments of those who stressed productivity, there being a nonnegligible incidence among them of the Blimpish view that the depression was, after all, simply the nemetic consequence of personal softness, “living beyond one's means,” and a lack of concern for the morrow.

• 1989, Isaac Asimov, Nemesis (1989), p 156 ― Nemesis will go right through the Solar System and have a perceptible effect on several of the planets. .. The Nemetic System is much smaller than the Solar System and therefore is held together more tightly.

• 2002, Kane X. Faucher; Modalities, logic, and the Cabala in Borges’ “The Theme of the Traitor and the Hero”. (Dossier: Borges: perplejidades filosoficas). Variaciones Borges, July 1, 2002; http://business.highbeam.com/434832/article-1G1-87916115/modalities-logic-and-cabala-borges-theme-traitor-and ― For Borges, history is nemetic – a measure of all history – while the particularities or singularities (the people who are affected by the repeating events) are anamnetic: the Platonico-Leibnizian doctrine of reminiscence.

• 2003, Julian Barnes; The seeds of rebellion; The Guardian, 28 March 2003 ― Moralists know that Hubris inevitably leads to Nemesis, but never before had the theory been given such literal expression. .. Why it went wrong we Nemetics never discovered.

• 2004, David Abram, Goa, page 146, ― Environmental groups and villagers keen to preserve the area's relative tranquillity secretly hope the nemetic powers of the mysterious old curse will not wane.

• 2009, Tim Rifat Interview with Jeff Rense; April 28, 2009; http://educate-yourself.org/cn/timrifat28apr09interview.shtml ― Obama has been placed into the archetypal template which fits into the nemetic group mind and he is being used as the ultimate thought weapon to drive the entire Western group mind into a mob consciousness prior to the Ashkenazy neo-Stalinist takeover.

• 2009, Simon Warner; Wainwright or wrong? No rock in Rufus opera, July 29, 2009, http://simonwarner.wordpress.com/2009/07/20/wainwright-or-wrong-rufus-opera-opens-and-closes ― a leering and unnerving creation by Jonathan Summers as the singer’s domineering mentor, suggestive, in lime green suit, of Batman’s nemetic Joker;

• Steve Mann, Jason Nolan, and Barry Wellman; Technomimesis and Sousveillance: Using Technology to Challenge Technosocial Surveillance; http://wearcam.org/cyborg_and_community/cyborgandcommunity.0.15short.html ― The mimetic act of holding a mirror to the social environment is something that Mann moves beyond written narrative to the actual use of similar surveillance techniques to “watch the watchers”, and this ability allows for the potential taking up of the nemetic role of “bringer of social justice” presented as an act of inquiry.

That proves that the standard classical derivational morphology of nemetic < nemesis is a recognized construct already in use in English, so you might as well use it.

Medium-term exists as a phrase, but it mostly refers to public finance. In this sense, it means:

2-10 years, usually used in reference to bonds. When used in technical analysis, usually a few weeks to a few months. Also called intermediate-term.

Intermediate-term, another option, is also primarily associated with finance. However, people who are aware of the adjectives "long-term" and "short-term" will understand that "medium-term" or "intermediate-term" is between the two.

For any of the adjectives, they describe a property of something. The consequences have the property of being long-term. So you don't need a preposition with them. Compare:

The [Noun] is [Adjective]

The cat is black

The consequences are long-term