We knoww from the Canon that each House at the Assassins Guild School has its own unique crest, but none are elaborated upon. What if...
This is another of those non-canonical House badges for the Guild of Assassins School, which are such fun to create. This one is for the Rt.Hon. Miss Alice Band's House, another of the four all-female School Houses. A word of explanation here. There is one of those irritating little continuity errors in the Discworld canon. In the Assassins' Guild Yearbook, and in the Discworld Companions, it is made perfectly clear that Alice Band is the Housemistress of Tump House. However, in the notes accomanying the Kidby illustration of Alice in "the Art of Discworld", Terry, in his own words, makes her Housemistress of the all-female Mantis house.
Now Tump House makes a kind of sense. We know from practice on Earth that once big businessmen make it, they tend to practice selective altruism, like spending some of that money on endowing an Oxbridge professorate, or psonsoring capital investment at a prestigious public school. This makes them look like philanthropists and diverts attention away from how they may have earnt the money in the first place. It is therefore entirely possible that Reacher Gilt, the man who had the Tump Tower built in Ankh-Morpork, endowed the Assassins' Guild school with enough money to fund a new House. Good publicity and it gets the Assassins onside and favourably disposed.
But. Gilt's disgrace and fall makes it very likely anyone associated with him would seek to break contact very quickly. Hence the re-naming of Tump House. By analogy with the other female Houses at the Assassins' School, which are named after creatures whose females are attitudinal towards the male (Raven, Black Widow and Scorpion), then another attitudinal female is the Preying Mantis - a creature that bites off the male's head just after sex, and then eats him. Thus:
Escutcheon divisée de bande gules bearing motto "Sororitas Deinque Misericordia". Superior: une mante attaquante (femme) couchant verte, on a field argent, le tete de la mante attaquante (homme) attrapé entre les dents.
Inferior: Une tour foudre-frappée argent sur une colline vert on a field bleu- cerulé.
Supporters: two mante attaquante rampant.
Motto: Tamen Usque Recurrit - She will Always Return.
Shield divided horizontally by a red band bearing the motto "Sororitas Deinque Misericordia". (Sisterhood Of Final Mercy).
Upper: a female preying mantis in green bearing the head of a male in her jaws against a white background.
Lower: a shattered fallen tower on a hill against a sky-blue background.
Shield supported by two upright preying mantises.
This design combines both the broken tower - the disgrace of Reacher Gilt and the symbolic fall of the Tump Tower - with the deed the preying mantis is most famous for.
It is believed Miss Alice Band personally designed the House Crest. People who believe they know something about her do not express surprise at this.