they're invisible, nothing else seems to allow summoning invisible monsters, and handling of such is a can of worms due to the visual messages when summoning ("A shadowy figure forms in the air before you [that you can't see].").
git-svn-id: https://crawl-ref.svn.sourceforge.net/svnroot/crawl-ref/trunk@5470 c06c8d41-db1a-0410-9941-cceddc491573
PLSXOISO66Y4475D2LMC7KMHZUWXNUBNFYMYFIVLOZKAOKUJYXMQC
thing_called = ((temp_rand > 8) ? MONS_WRAITH : // 64%
(temp_rand > 4) ? MONS_FREEZING_WRAITH : // 16%
(temp_rand > 1) ? MONS_SPECTRAL_WARRIOR // 12%
: MONS_SHADOW_WRAITH); // 8%
thing_called = ((temp_rand > 8) ? MONS_WRAITH : // 64%
(temp_rand > 3) ? MONS_FREEZING_WRAITH // 20%
: MONS_SPECTRAL_WARRIOR); // 16%