According to the gazetteer, Hyperborea is rife with such celestial phenomena as eclipses, comets, meteors, aurorae, and more.  Why not use these things to torment your players?  Celestial phenomena could either be added to a regular encounter table, or rolled separately (probably no more than a 1-in-12 daily chance, because some of these are pretty nasty).  When celestial phenomena is indicated, roll 3d6 on this table and hope for the sake of your players that you don't get Uncanny Vertigo or The Stars are Right.

CELESTIAL PHENOMENA TABLE (3d6)
 
03 – Uncanny Vertigo
04 – Weird Vibrations
05 – Accursed Moonbeam
06 – Shining Mist
07 – Star Stone
08 – Meteor Shower
09 – Stellar Conjunction
10 – Aurora
11 – Falling Star
12 – Lights in the Sky
13 – Mocking Star
14 – Lunar Eclipse
15 – Planetary Alignment
16 – Comet
17 – Wind from the Gulf
18 – The Stars are Right
 
Accursed Moonbeam: Night hours only.  One randomly-selected PC becomes lit by a sickly beam of pale moonlight.  Even if the PC takes shelter indoors, the insidious ray will somehow find its way inside within 1d3 rounds.  While lit by the moonbeam, the PC cannot sleep, cannot hide in shadows or move silently, is considered to be at -2 on TA and CA, and can be attacked at +2.  The sinister moonbeam fades away in 7 hours or at dawn, whichever comes first.
Aurora: Brilliant streamers of red, green, and purple light the sky above for 1d6 hours.  There is a 25% chance that the party will meet some otherworldly entity during this period.
Comet: A dazzling comet burns in the sky above Hyperborea for 5d4+10 days.  There is a 50% chance that all sorcerers will cast spells at +1d4 CA during this time; otherwise, all spells are cast at -1d4 CA.  There may be additional effects (e.g., interference with certain spells, or additional otherworldly encounters) at the referee’s discretion.
Falling Star: A single meteor flashes across the sky.  Each player should roll d20; the high roller spotted the falling star and is considered to be under a bless spell for the next 12 hours.
Lights in the Sky: 1d12 lights of various colors are seen dancing in the sky for 1d3 turns.  At the end of this period, there is a 50% chance that an otherworldly entity will be met.
Lunar Eclipse: The dread shadow of Hyperborea falls across one of its moons.  During the 1d3 hours of the eclipse, all sorcerers are considered to be at +2 CA for purposes of spell effects.
Meteor Shower: A dazzling array of meteors light up the sky, an omen of great significance.  The PC with the best sorcery saving throw must roll it.  If the saving throw succeeds, all members of the party are under a bless spell for 24 hours; if the saving throw fails, all are under a blight spell for the same period.
Mocking Star: One randomly-selected PC becomes aware that a particularly unwholesome star is leering and winking at him.  The PC must make a sorcery saving throw or lose one point each of INT and WIS for the next 12 hours, at which time the mocking star ceases to bedevil him.
Planetary Alignment: Planets are in a conjunction with Saturn of profound occult significance.  During the 1d3 hours of this alignment, all sorcerers are considered to be at +3 CA for purposes of spell effects.
Shining Mist: An area of shining white mist 1d6 miles across appears, centered on the PCs and lasting 1d8 days.  Effects on vision are the same as a fog cloud spell.  Movement is halved and the chance of becoming lost is doubled.  All random encounters within the mist will be with otherworldly entities.  When PCs leave the mist, they are 25% likely to have been transported 1d10x10 miles in a random direction from the point where they entered.
Star Stone: A meteorite is seen plunging to earth 1d4 miles from the PCs.  If recovered, the star stone is worth 1d12x1d10 thousand gold pieces at a major city; however, sorcerers, madmen, and otherworldly entities will certainly pursue the PCs as long as they have the stone.
Stellar Conjunction: A cryptic alignment of significant stars lasts 1d3 hours.  During this time, all sorcerers are considered to be at +1 CA for purposes of spell effects.
The Stars are Right: All the stars in the firmament align in an eons-delayed pattern of monumental occult significance, the effects of which last 1d20 days.  Evil sorcerers (only) have their CA doubled for purposes of spell effects, undead and daemons cannot be turned, and all random encounters are with otherworldly entities.  There may be additional game-changing effects at the referee’s option (e.g., Kthulhu rises from his frozen grave and ravages Hyperborea).
Uncanny Vertigo: The entire party becomes sickeningly aware that they are standing on a flat disk floating in the depths of the illimitable Black Gulf.  Each PC must roll a sorcery saving throw or be left retching and incapacitated for 1d6 hours.  If the roll on this saving throw is a 1, the PC actually falls off of Hyperborea and into the Black Gulf.
Weird Vibrations: The entire party is assailed by cacodaemoniacal melodies and the music of unholy spheres for 1d6 hours.  All PCs are at -2 on CA and TA, -2 to hit and damage, and -2 on saving throws.  In addition, each PC must make a sorcery saving throw each hour or lose 1d3 points of INT and WIS.  Any PC who falls below 3 in either INT or WIS goes violently insane until the daemonic music stops.  When the vibrations cease, INT and WIS are regained at the rate of 1d3 points per turn, but any PC who went temporarily insane must make a sorcery saving throw or suffer 1d3 permanent insanities.
Wind from the Gulf: Outdoors only.  A daemonic freezing wind blasts down on the party from directly above for 2d6 hours.  The entire party is considered to be under a blight spell during that period, and all attempts to turn undead are at -2 TA.  Any PC who fails to seek sturdy shelter (i.e., a cave or stone building) must make a death saving throw every hour or suffer 1d20 points of damage.  At the referee’s discretion, something may arrive from the Black Gulf during this period.